sfc: implement ndo_get_phys_port_name

Output is of the form p<port-number>.
Note that the port numbers don't necessarily map one-to-one to physical
 cages, partly because of 4x10G port modes on QSFP+ and partly because
 of hw/fw implementation details.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>