ath9k_hw: correct AR_PHY_SPECTRAL_SCAN register offset

We had 0x9912 but AR_PHY_SPECTRAL_SCAN is 0x9910. By using the
0x9912 we were making the hardware unresponsive. This allows us
to move forward with hardware reset on ar9271 on the ath9k_htc
driver.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>