ath9k: ar5008_hw_cmn_spur_mitigate: add missing mask_m & mask_p initialisation

by moving common code to ar5008_hw_cmn_spur_mitigate i forgot to move
mask_m & mask_p initialisation. This coused a performance regression
on ar9281.

Fixes: f911085f

 ("ath9k: split ar5008_hw_spur_mitigate and reuse common code in ar9002_hw_spur_mitigate.")
Reported-by: Gustav Frederiksen <lkml2017@openmailbox.org>
Tested-by: Gustav Frederiksen <lkml2017@openmailbox.org>
Cc: <stable@vger.kernel.org> # 4.2+
Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>