net: mv88e6xxx: Enable PORT_CONTROL_FORWARD_UNKNOWN for DSA-tagged CPU ports

Without it, a mv88e6131 switch will not forward incoming unicast
packets to the CPU port.

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>