net: dsa: mv88e6xxx: fix duplicate vlan warning

When setting VLANs on DSA switches, the VLAN is added to both the port
concerned as well as the CPU port by dsa_slave_vlan_add(), as well as
any DSA ports.  If multiple ports are configured with the same VLAN ID,
this triggers a warning on the CPU and DSA ports.

Avoid this warning for CPU and DSA ports.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>