mac80211: pass up beacons from external BSS when operating as AP

Beacons from external BSSes are required for updating overlapping BSS
info (i.e. ERP protection). Pass them up unconditionally.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>