SMP_Encrypt refactor

* key must be 16bit, so no need to pass it's length, and check it's
  value
* p_out being null is a bad call that should crash to be catched, not an
  error we should handle
* hash computation always succeed, don't return any errors

Test: existing unit tests cover it, i.e. AesCmacTest
Change-Id: Ifba8902f3f1a7be3c54dd7a33d9ff1e52ffd237b