ath9k: Make ANI CCK and OFDM error triggers band specific

The CCK and OFDM ANI error triggers are not mode specific but rather
band specific so just make use of the already available band from
ieee80211_conf.

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