ath9k: Fix ANI monitoring

The commit "ath9k_hw: improve ANI processing and rx desensitizing parameters"
changed various ANI operational parameters to address a specific
card/environment. This is not really applicable for other cards
in general usage.

As per internal documentation, lowering the immunity level can be
done only after 5 periods have passed and the CCK/OFDM errors are
below the low watermak threshold - which have been fixed at 300 and
400 respectively by the sytems team.

Raising the immunity level can be done when CCK/OFDM errors exceed
600 and 1000 (per second).

Set these values once during attach.

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