This is a partial revert of commit c295f983 ("net: mscc: ocelot:
switch from {,un}set to {,un}assign for tag_8021q CPU ports"), because
as it turns out, this isn't how tag_8021q CPU ports under a LAG are
supposed to work.

Under that scenario, all user ports are "assigned" to the single
tag_8021q CPU port represented by the logical port corresponding to the
bonding interface. So one CPU port in a LAG would have is_dsa_8021q_cpu
set to true (the one whose physical port ID is equal to the logical port
ID), and the other one to false.

In turn, this makes 2 undesirable things happen:

(1) PGID_CPU contains only the first physical CPU port, rather than both
(2) only the first CPU port will be added to the private VLANs used by
    ocelot for VLAN-unaware bridging

To make the driver behave in the same way for both bonded CPU ports, we
need to bring back the old concept of setting up a port as a tag_8021q
CPU port, and this is what deals with VLAN membership and PGID_CPU
updating. But we also need the CPU port "assignment" (the user to CPU
port affinity), and this is what updates the PGID_SRC forwarding rules.

All DSA CPU ports are statically configured for tag_8021q mode when the
tagging protocol is changed to ocelot-8021q. User ports are "assigned"
to one CPU port or the other dynamically (this will be handled by a
future change).
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>