rt2x00: Only update rssi average approximation on receiving beacon frames.

Restrict rssi average updating to beacon frames of the bssid the
interface is
associated with. Without this restriction, strong signals belonging to other
BSS, e.g. beacon frames coming from a nearby AP, would cause incorrectly high
rssi approximation values. This would then cause the link tuner to reduce
sensitivity, resulting in transmissions from the BSS associated to to be
missed.

Signed-off-by: Mattias Nissler <mattias.nissler@gmx.de>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>