ar9170: fix build when !CONFIG_PM

Fix this build error when CONFIG_PM is not set:
drivers/net/wireless/ath/ar9170/usb.c: In function 'ar9170_usb_probe':
drivers/net/wireless/ath/ar9170/usb.c:692:
	error: 'struct usb_device' has no member named 'reset_resume'

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>