net: dsa: Implement ndo_get_phys_port_name()

Return the physical port number of a DSA created network device using
ndo_get_phys_port_name().

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