mac80211: make beacon filtering per virtual interface

Due to firmware limitations, we may not be able to
support beacon filtering on all virtual interfaces.
To allow this in mac80211, introduce per-interface
driver capability flags that the driver sets when
an interface is added.

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