net: dsa: tag_sja1105: add compatibility with hwaccel VLAN tags

Check whether there is any hwaccel VLAN tag on RX, and if there is,
treat it as the tag_8021q header.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>