net: dsa: Implement ndo_get_phys_port_id

Implement ndo_get_phys_port_id() by returning the physical port number
of the switch this per-port DSA created network interface corresponds
to.

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