net: dsa: mv88e6xxx: include DSA ports in VLANs

DSA ports must be members of a VLAN in order to ensure frame bridging
between chained switch chips.

Thus tag them in addition to the CPU port when adding a VLAN, and skip
them when deleting a VLAN and reporting VLAN members.

Also use the UNMODIFIED egress policy, so that frames egress on these
ports as they ingress, tagged or untagged.

Fixes: 0d3b33e6 ("net: dsa: mv88e6xxx: add VLAN Load support")
Reported-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>