net: usb: asix: make use of mdiobus_get_phy and phy_connect_direct

In most cases we use own mdio bus, there is no need to create and store
string for the PHY address.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>