LinuxKPI: 802.11: support manual lladdr changes

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
Tested by: lwhsu
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46121