net: devlink: add warning for ndo_get_phys_port_name set when not needed

Currently if the driver registers devlink port instance, it should set
the devlink port attributes as well. Then the devlink core is able to
obtain physical port name itself, no need for driver to implement
the ndo. Once all drivers will implement devlink port registration,
this ndo should be removed. This warning guides new
drivers to do things as they should be done.

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>