ath5k: keep beacon RSSI average

Keep an exponentially weighted moving average of the beacon RSSI in our BSS.
It will be used by the ANI implementation.

The averaging algorithm is copied from rt2x00, Thanks :)

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>