ath9k_hw: Fix regression of register offset of AR9330/AR9340

The commit ce407afc introduced regression for AR9330/AR9340
register offsets. Some of the register offsets are common
for AR9330/AR9340/AR9485 except AR9380. Fix that.

Cc: stable@kernel.org [3.1.0+]
Cc: Senthil Balasubramanian <senthilb@qca.qualcomm.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>