ath9k: Add Bluetooth Coexistence 3-wire support

This patch adds 3-wire bluetooth coex support for AR9285.
This support can be enabled through btcoex_enable modparam.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>