mlxsw: spectrum: Set physical device for port netdevice

Do this so the sysfs has "device" link correctly set.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>