net: dsa: mv88e6xxx: Only reconfigure MAC when something changes

phylink will call the mac_config() callback once per second when
polling a PHY or a fixed link. The MAC driver is not supposed to
reconfigure the MAC if nothing has changed.

Make the mv88e6xxx driver look at the current configuration of the
port, and return early if nothing has changed.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>