ath9k: clean up ANI per-channel pointer checking

commit c24bd362 ("ath9k: Do not maintain ANI state per-channel")
removed per-channel handling, the code to check 'curchan' also
should be removed as never used.

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>