sfc: Add code to export port_num in netdev->dev_port

In the case where we have multiple functions (PFs and VFs), this
sysfs entry is useful to identify the physical port corresponding
to the function we are interested in.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>