LinuxKPI: 802.11: place lock asserts for key operation

Since b8dfc3ecf7031 we hold the wiphy lock over any key updates.
Assert that on function entry.

Given this invariant there is no need to recheck a kay under lock now,
as the first lookup already is under the lock, so simplify the code.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 284af342971d102702df950a2f1e5479eaabdf0b)