kernel/workq: Cleanup bespoke reschedule point

The work queue has a semi/non-standard reschedule point implemented
using k_yield(), with a check to see if the current thread is
preemptible.  Just call z_reschedule_unlocked(), it has this check
internally and is the intended API for this.

Really, this is only a half fix.  Ideally the schedule point and the
lock release should be atomic[1] via the more idiomatic
z_reschedule().  But that would take some surgery, so let's go with
the simpler cleanup first.

This also avoids having to duplicate logic that gets added to
reschedule points by an upcoming patch.

[1] So that they represent a condition variable and don't race at the
end. In this case the race is present but benign, since the only thing
we really want to know is that the queue thread gets a chance to run.
The only cost is an occasional duplicated/needless context switch if
two threads are racing on a submit.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
(cherry picked from commit 8d94967e)