kernel: Remove k_thread_priority_set() restriction

Removes the ISR restriction from k_thread_priority_set().
Since the first commit, the routine k_thread_priority_set() has
had a restriction preventing it from being called from within
the context of an ISR. As there does not (any longer) appear to
be a reason why the restriction exists, it is being removed.

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