net: dsa: mv88e6xxx: support VLAN filtering

Implement port_vlan_filtering in the driver to toggle the related port
802.1Q mode between DISABLED and SECURE, on user request.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>