kernel - Clean up if_bridge bif_state tests

kernel - Clean up if_bridge bif_state tests

* bif_state is only valid when IFBIF_STP is set, adjust two bits of
  code that were using bif_state unconditionally.

* This is a semi-operational change because bif_state's default value
  when IFBIF_STP is not set resulted in correct operation anyway.

  However, setting STP and then clearing it on a sub-interface could
  cause problems with stale state.