ath9k: Remove redundant my beacon check at ath_rx_ps_beacon

Make use of the rx status's is_mybeacon in order to avoid
redundant memcmp.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>