b43: verify sta_notify mac80211 callback

This helps verify that nothing bad is going on in mac80211,
it is unfortunately not possible to implement this generically
in mac80211 easily because there we can't assume that we only
have a single vif which b43 currently can assume.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>