net: usb: asix: ax88772: add missing stop

Add missing stop and let phylib framework suspend attached PHY.

Fixes: e532a096 ("net: usb: asix: ax88772: add phylib support")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>