Cleaning up and improving locking in if_bwn.

* Removing all BWN_LOCK, BWN_UNLOCK and BWN_ASSERT_LOCKED that were left.
  Substituting some of the BWN_ASSERT_LOCKED with wlan_assert_serialized().

* Adding wlan_serialize_exit()/wlan_serialize_enter() brackets around
  taskqueue_drain and taskqueue_free calls.