ath9k_hw: clean up ANI state handling

ANI state is kept per channel, so instead of keeping an array of ANI states
with an arbitrary size of 255, move the ANI state into the channel struct.

Move some config settings that are not per-channel out of
the per-channel struct to save some memory.

With those changes, ath9k_ani_restart_old and ath9k_ani_restart_new can
be merged into a single function.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>