ath9k_htc: Fix beacon miss under heavy load

Transmission of beacons becomes erratic when TX load
is high, since the latency involved in the generation
of a SWBA interrupt on the target to the actual sending
of a beacon is quite high for USB devices.

Fix this by adjusting the beacon response time.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>