lan78xx: relocate mdix setting to phy driver

Relocate mdix code to phy driver to be called at config_init().

Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>