net: dsa: tag_8021q: introduce a vid_is_dsa_8021q helper

This function returns a boolean denoting whether the VLAN passed as
argument is part of the 1024-3071 range that the dsa_8021q tagging
scheme uses.

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>