ath9k: Fix a DMA latency issue for Intel Pinetrail platforms.

Throughput was severely affected in Intel Pinetrail platforms
because of a DMA problem in C3 state. This patch fixes this
issue.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
CC: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>