ath9k: enable ANI to help with noisy environments

This enables Adaptive Noise Immunity (ANI) on ath9k.
ANI is as algorithm designed to minimize the detrimental
effects of time-varying interferences. This should
help with throughput in noisy environments. To use
ANI we re-enable the MIB interrupt. Since ANI works
on a timer and updates the noise floor we take
advantage of this and also report a non-static noise
floor now to mac80211.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: Jouni Malinen <Jouni.Malinen@Atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>