net: dsa: fix bridge support for drivers without port_bridge_flags callback

Starting with patch:
a8b659e7 ("net: dsa: act as passthrough for bridge port flags")

drivers without "port_bridge_flags" callback will fail to join the bridge.
Looking at the code, -EOPNOTSUPP seems to be the proper return value,
which makes at least microchip and atheros switches work again.

Fixes: 5961d6a1 ("net: dsa: inherit the actual bridge port flags at join time")
Signed-off-by: NOleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dsa/port.c

@@ -550,7 +550,7 @@ int dsa_port_bridge_flags(const struct dsa_port *dp,
 	struct dsa_switch *ds = dp->ds;
 
 	if (!ds->ops->port_bridge_flags)
-		return -EINVAL;
+		return -EOPNOTSUPP;
 
 	return ds->ops->port_bridge_flags(ds, dp->index, flags, extack);
 }