net: dsa: mv88e6xxx: use port 802.1Q mode Secure

This commit changes the 802.1Q mode of each port from Disabled to
Secure. This enables the VLAN support, by checking the VTU entries on
ingress.

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