net: dsa: lantiq_gswip: suppress -EINVAL errors for bridge FDB entries added to the CPU port

The blamed commit and others in that patch set started the trend
of reusing existing DSA driver API for a new purpose: calling
ds->ops->port_fdb_add() on the CPU port.

The lantiq_gswip driver was not prepared to handle that, as can be seen
from the many errors that Daniel presents in the logs:

[  174.050000] gswip 1e108000.switch: port 2 failed to add fa:aa:72:f4:8b:1e vid 1 to fdb: -22
[  174.060000] gswip 1e108000.switch lan2: entered promiscuous mode
[  174.070000] gswip 1e108000.switch: port 2 failed to add 00:01:02:03:04:02 vid 0 to fdb: -22
[  174.090000] gswip 1e108000.switch: port 2 failed to add 00:01:02:03:04:02 vid 1 to fdb: -22
[  174.090000] gswip 1e108000.switch: port 2 failed to delete fa:aa:72:f4:8b:1e vid 1 from fdb: -2

The errors are because gswip_port_fdb() wants to get a handle to the
bridge that originated these FDB events, to associate it with a FID.
Absolutely honourable purpose, however this only works for user ports.

To get the bridge that generated an FDB entry for the CPU port, one
would need to look at the db.bridge.dev argument. But this was
introduced in commit c269336 ("net: dsa: request drivers to perform
FDB isolation"), first appeared in v5.18, and when the blamed commit was
introduced in v5.14, no such API existed.

So the core DSA feature was introduced way too soon for lantiq_gswip.
Not acting on these host FDB entries and suppressing any errors has no
other negative effect, and practically returns us to not supporting the
host filtering feature at all - peacefully, this time.

Fixes: 10fae4a ("net: dsa: include bridge addresses which are local in the host fdb list")
Reported-by: Daniel Golle <daniel@makrotopia.org>
Closes: https://lore.kernel.org/netdev/aJfNMLNoi1VOsPrN@pidgin.makrotopia.org/
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250918072142.894692-3-vladimir.oltean@nxp.com
Tested-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: vladimiroltean

drivers/net/dsa/lantiq_gswip.c

@@ -1368,8 +1368,9 @@ static int gswip_port_fdb(struct dsa_switch *ds, int port,
 	int i;
 	int err;
 
+	/* Operation not supported on the CPU port, don't throw errors */
 	if (!bridge)
-		return -EINVAL;
+		return 0;
 
 	for (i = max_ports; i < ARRAY_SIZE(priv->vlans); i++) {
 		if (priv->vlans[i].bridge == bridge) {