LinuxKPI: 802.11: implement mtx support for ieee80211_iterate_interfaces

Implement the lockdep_assert_wiphy call for
ieee80211_iterate_active_interfaces_mtx() to avoid a warning when
used and to make sure callers comply with assumptions.

Leave an "IMPROVE" note as we can likely switch another of the multiple
callers to RCU.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days