kernel: sched: Fix validation of priority levels

A priority value cannot be simultaneously higher than the maximum
possible value and smaller than the minimum value.  Rewrite the
_VALID_PRIO() macro as a function so that this if either of these
invariants are invalid, the priority is considered invalid.

Coverity-CID: 182584
Coverity-CID: 182585
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>