cpu/cc2538/spi: Fix compiler warning of possible uninitialized variable

[DipSwitch]

During the build test of #4413 the following errors where shown:

bindist:openmote-cc2538:
Building application "bindist" for "openmote-cc2538" with MCU "cc2538".

/data/riotbuild/cpu/cc2538/periph/spi.c: In function 'ssi_flush_input':
/data/riotbuild/cpu/cc2538/periph/spi.c:224:12: error: 'tmp' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     return tmp;
            ^
cc1: all warnings being treated as errors


bindist:remote:
Building application "bindist" for "remote" with MCU "cc2538".

/data/riotbuild/cpu/cc2538/periph/spi.c: In function 'ssi_flush_input':
/data/riotbuild/cpu/cc2538/periph/spi.c:224:12: error: 'tmp' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     return tmp;
            ^
cc1: all warnings being treated as errors

bindist:cc2538dk:
Building application "bindist" for "cc2538dk" with MCU "cc2538".

/data/riotbuild/cpu/cc2538/periph/spi.c: In function 'ssi_flush_input':
/data/riotbuild/cpu/cc2538/periph/spi.c:224:12: error: 'tmp' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     return tmp;
            ^
cc1: all warnings being treated as errors

This fixes that.

[DipSwitch]

Although maybe flush doesn't need to return the tmp value?

[DipSwitch]

Is the compiler updated on Murdock? Since this change got nothing to do with this error...

gnrc_border_router:native:
Building application "gnrc_border_router" for "native" with MCU "native".

/data/riotbuild/sys/net/gnrc/network_layer/sixlowpan/iphc/gnrc_sixlowpan_iphc.c: In function 'iphc_nhc_udp_encode':
/data/riotbuild/sys/net/gnrc/network_layer/sixlowpan/iphc/gnrc_sixlowpan_iphc.c:569:33: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
         for (int i = nhc_len; i < (udp->size - diff); i++) {
                                 ^
cc1: all warnings being treated as errors

zep:native:
Building application "zep" for "native" with MCU "native".

/data/riotbuild/sys/net/gnrc/network_layer/sixlowpan/iphc/gnrc_sixlowpan_iphc.c: In function 'iphc_nhc_udp_encode':
/data/riotbuild/sys/net/gnrc/network_layer/sixlowpan/iphc/gnrc_sixlowpan_iphc.c:569:33: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
         for (int i = nhc_len; i < (udp->size - diff); i++) {
                                 ^
cc1: all warnings being treated as errors

[kYc0o]

@kaspar030 any ideas on this? It appears maybe that other PR broke this...

[kYc0o]

Fixed in #5360

[kYc0o]

ACK and go!

[kYc0o]

Backport on #5357

[miri64]

please remove the label, when a backport was opened, so we can see which PRs still require one ;-)