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.

MFC after: 3 days
Reviewed by: cc
Differential Revision: https://reviews.freebsd.org/D43512