net80211: introduce (*iv_update_bss)()

Introduce (*iv_update_bss)() with a default implementation to allow
drivers to overload/intercept the time when we swap iv_bss.

This helps firmware based drivers to synchronize state with firmware.
Otherwise, for some state changes, we begin with one ni (and in
LinuxKPI lsta) and try to finish with another ni (and a new lsta
in different state) and may no longer have access to the previous state.
This also saves us from constantly checking for ni changes complicating
code.

No functional changes intended.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
X-MFC: move (*iv_update_bss) to spare area