ath9k: split ar5008_hw_spur_mitigate and reuse common code in ar9002_hw_spur_mitigate.

ath9k: split ar5008_hw_spur_mitigate and reuse common code in ar9002_hw_spur_mitigate.

[ar5008 and ar9002]_hw_spur_mitigate have big portion of identical code.
This patch will move common part of ar5008_hw_spur_mitigate to
ar5008_hw_cmn_spur_mitigate and reuse it in ar9002_hw_spur_mitigate.

As noticed by Joe Perches I reuse ar9002_hw_spur_mitigate (const) version of
declarations for pilot_mask_reg and chan_mask_reg.

There should be no other difference with original code.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>