dsa: do not support ndo_get_phys_port_name for non-legacy ports

Since each non-legacy slave has its own devlink port instance
correctly set, rely on devlink core to generate correct phys port name.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>