net: dsa: mv88e6xxx: exclude all ports in new VLAN

Now that the DSA core adds the CPU and DSA ports itself to the new VLAN
entry, there is no need to include them as members of this VLAN when
initializing a new VTU entry.

As of now, initialize a new VTU entry with all ports excluded.

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>