mac80211: don't return beacons when mesh is disabled

When mesh is disabled, mac80211 was returning
beacons with an empty mesh ID. That isn't
desirable, even if drivers shouldn't be trying
to get beacons to start with.

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