dsa: mv88e6xxx: Don't poll forced interfaces for state changes

When polling for link status, don't consider ports which have a forced
link. Such ports don't monitor their phy or may not even have a phy.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>