net: dsa: mv88e6xxx: Fix masking of egress port

Add missing ~ to the usage of the mask.

Reported-by: Kevin Benson <Kevin.Benson@zii.aero>
Reported-by: Chris Healy <Chris.Healy@zii.aero>
Fixes: 5c74c54c

 ("net: dsa: mv88e6xxx: Split monitor port configuration")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>