ath9k: Add power save wrappers and modularize hw_pll handler

We should protect hw_pll handler with power save wrappers and
also modularize hw_pll handler properly for better readability.

Also add a debug message to track chip resets on pll hang condition.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>