ath9k: reset noiseimmunity level to default

After the chip reset, the noise immunity levels are restored with
history values. If the immunity levels are lower than the defaults,
lets start with the optimal values.

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