net: dsa: tag_sja1105: implement sub-VLAN decoding

Create a subvlan_map as part of each port's tagger private structure.
This keeps reverse mappings of bridge-to-dsa_8021q VLAN retagging rules.

Note that as of this patch, this piece of code is never engaged, due to
the fact that the driver hasn't installed any retagging rule, so we'll
always see packets with a subvlan code of 0 (untagged).

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>