ath9k_hw: Enable hw PLL power save for AR9462

This reduced the power consumption to half in full and network sleep.

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