net: dsa: mt7530: don't change PVC_EG_TAG when CPU port becomes VLAN-aware

Frank reports that in a mt7530 setup where some ports are standalone and
some are in a VLAN-aware bridge, 8021q uppers of the standalone ports
lose their VLAN tag on xmit, as seen by the link partner.

This seems to occur because once the other ports join the VLAN-aware
bridge, mt7530_port_vlan_filtering() also calls
mt7530_port_set_vlan_aware(ds, cpu_dp->index), and this affects the way
that the switch processes the traffic of the standalone port.

Relevant is the PVC_EG_TAG bit. The MT7530 documentation says about it:

EG_TAG: Incoming Port Egress Tag VLAN Attribution
0: disabled (system default)
1: consistent (keep the original ingress tag attribute)

My interpretation is that this setting applies on the ingress port, and
"disabled" is basically the normal behavior, where the egress tag format
of the packet (tagged or untagged) is decided by the VLAN table
(MT7530_VLAN_EGRESS_UNTAG or MT7530_VLAN_EGRESS_TAG).

But there is also an option of overriding the system default behavior,
and for the egress tagging format of packets to be decided not by the
VLAN table, but simply by copying the ingress tag format (if ingress was
tagged, egress is tagged; if ingress was untagged, egress is untagged;
aka "consistent). This is useful in 2 scenarios:

- VLAN-unaware bridge ports will always encounter a miss in the VLAN
  table. They should forward a packet as-is, though. So we use
  "consistent" there. See commit e045124e ("net: dsa: mt7530: fix
  tagged frames pass-through in VLAN-unaware mode").

- Traffic injected from the CPU port. The operating system is in god
  mode; if it wants a packet to exit as VLAN-tagged, it sends it as
  VLAN-tagged. Otherwise it sends it as VLAN-untagged*.

*This is true only if we don't consider the bridge TX forwarding offload
feature, which mt7530 doesn't support.

So for now, make the CPU port always stay in "consistent" mode to allow
software VLANs to be forwarded to their egress ports with the VLAN tag
intact, and not stripped.

Link: https://lore.kernel.org/netdev/trinity-e6294d28-636c-4c40-bb8b-b523521b00be-1674233135062@3c-app-gmx-bs36/
Fixes: e045124e

 ("net: dsa: mt7530: fix tagged frames pass-through in VLAN-unaware mode")
Reported-by: Frank Wunderlich <frank-w@public-files.de>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230205140713.1609281-1-vladimir.oltean@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>