net: dsa: mv88e6xxx: Disable Message Port bit for CPU port

Datasheet says that the Message Port bit should not be set for the CPU port.
Having it set causes DSA tagged packets to be sent to the CPU port roughly
every 30 seconds. Those packets are the same as real packets forwarded between
switch ports if the switch is configured for switching between multiple ports.
The packets are then bridged by the software bridge, resulting in duplicated
packets on the network.

Reported-by: Andrew Lunn <andrew@lunn.ch>
Cc: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>