net: usb: asix: ax88772: add generic selftest support

With working phylib support we are able now to use generic selftests.

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>