ath9k_hw: fix setting lower noise immunity values

Commit af1e8a6f

 "ath9k: reset noiseimmunity level to default" was supposed
to ensure that the default noise immunity level is above the INI
values, however it prevents setting lower noise immunity values altogether.
Fix this by moving the checks to the right function.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>