net: dsa: mv88e6xxx: define membership on VLAN add

Define the target port membership of the VLAN entry in
mv88e6xxx_port_vlan_add where ds is scoped.

Allow the DSA core to call later the port_vlan_add operation for CPU or
DSA ports, by using the Unmodified membership for these ports, as in the
current behavior.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>