kernel: Fix k_wakeup() exit paths

z_reschedule() already has a check to determine if it is called from
the context of an ISR--no need to duplicate it in k_wakeup().
Furthermore, if the target thread is not sleeping, there is no need
to reschedule and we can do a fast return.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>