mac80211: mesh STAs only process mesh beacons

Before, a mesh STA would execute some code on behalf of AP or IBSS
beacons. Since the mesh stack currently does not consider anything but
other mesh STAs interesting, limit processing to just these and save a
little overhead.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>