mac80211: count authorized stations per BSS

Currently, each AP interface will send multicast
traffic if any interface has a station entry even
if that station entry is allocated only. With the
new station state management we can easily fix it
by adding a counter that counts each authorized
station only and send multicast traffic only when
the correct interface has at least one authorized
station.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>