Fix polarity error in PWM.setup() for Linux 4.9

[pdp7]

For issue #170.

Commit 8e08a40 moved pwm_set_polarity() after pwm_set_frequency() and pwm_set_duty_cycle(). PWM.start() now runs OK after reboot.

Output from running test:

    debian@beaglebone:~/ssh/adafruit-beaglebone-io-python$ sudo python test/issue170-pwm.py; journalctl -p debug -t python --since "1 min ago"
    -- Logs begin at Tue 2017-10-31 06:56:00 UTC, end at Tue 2017-10-31 07:36:39 UTC. --
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: version <unknown> initialized
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: P9_14, 50.000000, 2000.000000, 1
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: lookup_exported_pwm: couldn't find 'P9_14'
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: initialize_pwm: OK
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: set_pin_mode() :: Pinmux file /sys/devices/platform/ocp/ocp:P9_14
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: set_pin_mode() :: Set pinmux mode to pwm for P9_14
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: P9_14, /sys/devices/platform/ocp/48302000.epwmss/48302
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: P9_14, /sys/devices/platform/ocp/48302000.epwmss/48302
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_setup: P9_14 OK
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: call pwm_set_frequency(key=P9_14 freq=2000.000000)
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_set_frequency: P9_14 2000.000000 OK
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: call pwm_set_duty_cycle(key=P9_14 duty=50.000000)
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_set_duty_cycle: P9_14 50.000000 OK
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: call pwm_set_polarity(key=P9_14 polarity=1)
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_set_polarity: P9_14 1 OK
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: write(pwm->enable_fd, buffer=1, len=1)
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: P9_14 OK
    Oct 31 07:35:57 beaglebone python[1997]: [40B blob data]
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: P9_14, 50.000000, 2000.000000, 0
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: lookup_exported_pwm: couldn't find 'P9_14'
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: set_pin_mode() :: Pinmux file /sys/devices/platform/ocp/ocp:P9_14
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: set_pin_mode() :: Set pinmux mode to pwm for P9_14
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: P9_14, /sys/devices/platform/ocp/48302000.epwmss/48302
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: P9_14, /sys/devices/platform/ocp/48302000.epwmss/48302
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_setup: P9_14 OK
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: call pwm_set_frequency(key=P9_14 freq=2000.000000)
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_set_frequency: P9_14 2000.000000 OK
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: call pwm_set_duty_cycle(key=P9_14 duty=50.000000)
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_set_duty_cycle: P9_14 50.000000 OK
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: call pwm_set_polarity(key=P9_14 polarity=0)
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_set_polarity: P9_14 0 OK
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: write(pwm->enable_fd, buffer=1, len=1)
    Oct 31 07:35:57 beaglebone python[1997]: Adafruit_BBIO: pwm_start: P9_14 OK
    Oct 31 07:35:57 beaglebone python[1997]: [40B blob data]

[pdp7]

pytest runs OK:

debian@beaglebone:~/ssh/adafruit-beaglebone-io-python/test$ sudo /opt/scripts/tools/version.sh 
git:/opt/scripts/:[6c2688b0be448b7bb9ca18849b430d496a84acb4]
eeprom:[A335BNLT000C3014BBBK1316]
model:[TI_AM335x_BeagleBone_Black]
dogtag:[BeagleBoard.org Debian Image 2017-10-17]
bootloader:[microSD-(push-button)]:[/dev/mmcblk0]:[U-Boot 2017.09-00002-g0f3f1c7907]
bootloader:[eMMC-(default)]:[/dev/mmcblk1]:[U-Boot 2017.09-rc2-00002-g84a7f2]
kernel:[4.9.57-ti-r71]
nodejs:[v6.11.4]
uboot_overlay_options:[enable_uboot_overlays=1]
uboot_overlay_options:[uboot_overlay_pru=/lib/firmware/AM335X-PRU-RPROC-4-4-TI-00A0.dtbo]
uboot_overlay_options:[enable_uboot_cape_universal=1]
pkg:[bb-cape-overlays]:[4.4.20171013.2-0rcnee1~stretch+20171013]
pkg:[bb-wl18xx-firmware]:[1.20170829-0rcnee1~stretch+20170829]
pkg:[firmware-ti-connectivity]:[20170823-1rcnee0~stretch+20170830]

debian@beaglebone:~/ssh/adafruit-beaglebone-io-python/test$ sudo python -mpytest 
================================================== test session starts ==================================================
platform linux2 -- Python 2.7.13, pytest-3.2.3, py-1.4.34, pluggy-0.4.0
rootdir: /home/debian/ssh/adafruit-beaglebone-io-python, inifile:
collected 58 items                                                                                                       

test_adc.py ......
test_gpio_input.py ..
test_gpio_output.py ......
test_gpio_setup.py ..........
test_led.py ..
test_pwm_setup.py ...........................
test_spi.py ..
test_uart.py ...

============================================== 58 passed in 13.53 seconds ===============================================