ath9k: Initialize the priority gpio for BT coex 3-wire

Oops, a stupid mistake in the original patch which adds coex 3-wire
support. Bluetooth priority gpio needs to be gpio 7.

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