ath9k: Do not maintain ANI state per-channel

ANI state can be maintained globally instead of per-channel.
This reduces memory usage and since default values are used
during a scan run, per-channel state is not required.

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