ath9k: fix compile error in ahb.c

drivers/net/wireless/ath9k/ahb.c: In function 'ath_ahb_probe':
drivers/net/wireless/ath9k/ahb.c:136: error: 'aphy' undeclared (first
use in this function)

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>