Revert "ath9k_hw: Fix incorrect spur_freq_sd for AR9003"

This reverts commit a844adfd.
The commit a844adfd is degrading rx sensitivity of lower rate in
HT40 mode and it is confirmed that reverting the change is
improving rx sensitivity.

spur_freq_sd (for self-corr in AGC) is defined with respect to the
center of each 20MHz channel while spur_phase_delta (for self-corr
in Rx and spur data filter) is defined with respect to the center
of current RF channel.

So in short, we need to subtract spur_freq_sd (for self-corr in AGC)
by the offset between the center of primary20 and the center of RF
channel in SW. This offset could be +/10 MHz for dynamic 40.

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