ar9170usb: module link in sysfs

Andrey Yurovsky reported that the driver forwarded erroneously the
parent device structure instead of the real thing, which of course
led to some dodgy sysfs links (at least?).

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>