ath9k_hw: Add a function to read sqsum_dvc.

Add a function to observe the delta VC of BB_PLL.
For a good chip, the sqsum_dvc is below 2000.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>