libertas_usb: use USB interface as parent device

Currently, "udevadm info -a -p /sys/class/net/wlan0" doesn't mention
the usb8xxx or libertas driver anywhere. This makes writing udev rules
a bit uncomfortable.

Using the USB interface as the parent device corrects the hierarchy.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>