mlxsw: Remember untagged VLANs

When a vlan is been configured, remeber the untagged mode of the vlan.
When displaying the list of configured VLANs, show the untagged attribute.

Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>