Some drivers might report that they are unable to bridge ports by
returning -EOPNOTSUPP, but still wants to override extack message.
In order to do so, in dsa_slave_changeupper(), if port_bridge_join()
returns -EOPNOTSUPP, check if extack message is set and if so, do not
override it.
Reviewed-by: Vladimir Oltean <olteanv@xxxxxxxxx>
Signed-off-by: Clément Léger <clement.leger@xxxxxxxxxxx>