net: dsa: b53: keep CPU port always tagged again

The Broadcom management header does not carry the original VLAN tag
state information, just the ingress port, so for untagged frames we do
not know from which VLAN they originated.

Therefore keep the CPU port always tagged except for VLAN 0.

Fixes the following setup:

$ ip link add br0 type bridge vlan_filtering 1
$ ip link set sw1p1 master br0
$ bridge vlan add dev br0 pvid untagged self
$ ip link add sw1p2.10 link sw1p2 type vlan id 10

Where VID 10 would stay untagged on the CPU port.

Fixes: 2c32a3d3 ("net: dsa: b53: Do not force CPU to be always tagged")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20250429201710.330937-3-jonas.gorski@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>