tests: drivers: can: api: fix test_set_timing_data_while_started test

Change the test_set_timing_data_while_started test to actually attempt to
set the data phase timing parameters instead of attempting to set the
nominal/arbitration timing parameters.

Update test descriptions to explicitly mention data phase timing.

Fixes: #59046

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>