LinuxKPI: 802.11: adjust locking

Split up the lhw lock and the scan lock. The latter is a mtx
while the former changes from mtx to sx as mac80211 downcalls may
sleep (and the ic lock is not usable in that case either and a larger
project to fix).
This will also enforce some lookups under lock (mostly scan) as well
as general protection for more compat code and avoid a possible
deadlock with one of the upcoming callbacks from driver into the
compat code.

Sponsored by: The FreeBSD Foundation
MFC after: 7 days