net: dsa: mv88e6xxx: Split monitor port configuration

Separate the configuration of the egress and ingress monitor port.
This allows the port mirror functionality to do ingress and egress
port mirroring to separate ports.

Signed-off-by: Iwan R Timmer <irtimmer@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>