ath9k: use ath_is_mybeacon

This patch will also change behavior of rx_beacons statistic.
Instead of collecting all received beacons, it will collect only
ours. This, IMO make more sense, since for troubleshooting we will
need to know count of our beacons, or both.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>