CAN ISO-TP implementation not using local work queue

[martinjaeger]

Describe the bug

A local k_work_q is defined in isotp.c, but the work is always submitted to the system work queue using k_work_submit.

Just replacing the calls with k_work_submit_to_queue resulted in failing unit tests on nucleo_l452re.

Implementation test:

*** Booting Zephyr OS build zephyr-v2.5.0-3125-gf4f9fcc13ba5  ***
Running test suite isotp
===================================================================
START - test_bind_unbind
E: Got a frame but was not yet ready for a new one

    Assertion failed at WEST_TOPDIR/zephyr/tests/subsys/canbus/isotp/implementation/src/main.c:79: get_sf_net: (remaining_len >= 0 is false)
recv returned -8
 FAIL - test_bind_unbind in 0.31 seconds
===================================================================
START - test_send_receive_net_sf
E: No buffer for FF left

    Assertion failed at WEST_TOPDIR/zephyr/tests/subsys/canbus/isotp/implementation/src/main.c:218: test_send_receive_net_sf: (ret not equal to 0)
Bind returned -11
 FAIL - test_send_receive_net_sf in 0.17 seconds

...

Conformance test:

...

===================================================================
START - test_receive_data_blocks
 PASS - test_receive_data_blocks in 1.143 seconds
===================================================================
START - test_send_timeouts
E: Reception of next FC has timed out
E: Got unexpected PDU

    Assertion failed at WEST_TOPDIR/zephyr/tests/subsys/canbus/isotp/conformance/src/main.c:138: send_complette_cb: (error_nr not equal to expected_err_nr)
Unexpected error nr. expect: -2, got -6
 FAIL - test_send_timeouts in 1.24 seconds
===================================================================

Impact

It is annoying that the stack space is reserved, but not actually used. And it's not obvious at the moment why the local work queue does not work. There must be something seriously wrong.