net: ethernet: ixp4xx: Retire ancient phy retrieveal

This driver was using a really dated way of obtaining the
phy by printing a string and using it with phy_connect().
Switch to using more reasonable modern interfaces.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>