ath9k: Store noise immunity values across scanning

CCK/OFDM noise immunilty values are always reset to defaults
during bgscan. This could affect the link quality and
performance when the STA is associated in a noisy channel.
So do not override the learned values across the scanning.

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