mac80211: don't drop mesh peering frames from unknown STA

Previously, mesh peering frames from a STA without a station
entry were being dropped.

Mesh Peering Open and other frames (WLAN_CATEGORY_SELF_PROTECTED)
are valid mesh peering frames even if received from a yet unknown
station; the STA entry will be created in mesh_peer_init later.

The problem didn't occur previously since both STAs receive each
other's beacons which created the STA entry. However, this causes
an unnecessary delay and beacons might not be received if either
node is in PS mode.

Signed-off-by: Marco Porsch <marco.porsch@etit.tu-chemnitz.de>
[reword commit log a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>