net: dsa: mediatek: combine MediaTek tag with VLAN tag

In order to let MT7530 switch can recognize well those egress packets
having both special tag and VLAN tag, the information about the special
tag should be carried on the existing VLAN tag. On the other hand, it's
unnecessary for extra handling for ingress packets when VLAN tag is
present since it is able to put the VLAN tag after the special tag and
then follow the existing way to parse.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>