Allow a user to change the "ether" address by ifconfig while a VAP is
not UP. Compared to net80211 (given we have no callback) we register
an eventhandler per-vif (a global one would force us to use hacks to
derive if a vap is indeed also a lkpi_80211 vif).
Sponsored by: The FreeBSD Foundation
PR: 277356
MFC after: 3 days