ath9k_hw: Fix incorrect spur_freq_sd for AR9003

Spur frequency was incorrectly computed with 10Mhz offset
which could cause the filter would not notch out the spur
and also this could improve rx sensitivity in HT40.

Cc: Madhan Jaganathan <madhanj@qca.qualcomm.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>