ath9k_hw: Change DCU backoff thresh for AR9340

By changing DCU backoff threshold for AR9340 to 1, helps to
reduce rx overrurns seen while running bidirectional traffic.

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