net: dsa: sja1105: implement VLAN retagging for dsa_8021q sub-VLANs

Expand the delta commit procedure for VLANs with additional logic for
treating bridge_vlans in the newly introduced operating mode,
SJA1105_VLAN_BEST_EFFORT.

For every bridge VLAN on every user port, a sub-VLAN index is calculated
and retagging rules are installed towards a dsa_8021q rx_vid that
encodes that sub-VLAN index. This way, the tagger can identify the
original VLANs.

Extra care is taken for VLANs to still work as intended in cross-chip
scenarios. Retagging may have unintended consequences for these because
a sub-VLAN encoding that works for the CPU does not make any sense for a
front-panel port.

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>