ath9k: Fix build error in ath_reset_internal

drivers/net/wireless/ath/ath9k/main.c:299 ath_reset_internal()
error: we previously assumed 'hchan' could be null (see line 293)

Cc: Felix Fietkau <nbd@openwrt.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>