net: dsa: mv88e6xxx: Make sure to configure ports with external PHYs

The MV88E6xxx can have external PHYs attached to certain ports and those
PHYs could even be on different MDIO bus than the one within the switch.
This patch makes sure that ports with such PHYs are configured correctly
according to the information provided by the PHY.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>