net80211: make sure calls to (*iv_update_bss)() are locked

It turned out thare various calls into (*iv_update_bss)(), that is
direct changes to vap->iv_bss in the old days, happened without
synchronisation.

Use locking assertions to document the requirement or status quo
at some callers given ic locking will eventually have to be dealt
with.

Reviewed by: cc
Differential Revision: https://reviews.freebsd.org/D43512

(cherry picked from commit 49619f73151aeaca4cef5adf631253da04a46e19)