net: usb: asix: add error handling for asix_mdio_* functions

This usb devices can be removed at any time, so we need to forward
correct error value if device was detached.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>