mlxsw: Remove ndo_get_phys_port_name implementation

Rely on the previously introduced fallback and let the core call
devlink directly in order to get the physical port name.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>