ath: Fix smatch warning

drivers/net/wireless/ath/main.c:88 ath_printk()
	 error: we previously assumed 'common->hw' could be null (see line 82)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>