net: dsa: mv88e6xxx: Allow dsa and cpu ports in multiple vlans

Ports with the same VLAN must all be in the same bridge. However the
CPU and DSA ports need to be in multiple VLANs spread over multiple
bridges. So exclude them when performing this test.

Fixes: b2f81d30

 ("net: dsa: add CPU and DSA ports as VLAN members")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>