kernel: pipes: Change method of unpending waiters

By the time the working list of readers/writers is processed, it is
possible that waiting reader/writer being processed had timed out
and is no longer on the wait queue. As such, we can not blindly
wake the next thread as that next thread might not be the thread we
had just been processing.

To address this, the calls to z_sched_wake() have been replaced
with z_unpend_thread() and z_ready_thread() so that a specific
thread can be safely targeted for waking.

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