ath9k_hw: Fix init mode register regression

The commit 172805ad
overwirtes additional clock settings of AR9330 to
all AR9300 chips.

Cc: stable@kernel.org
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>