sfc: Store port number in private data, not net_device::dev_id

We should not use net_device::dev_id to indicate the port number, as
this affects the way the local part of IPv6 addresses is normally
generated.

This field was intended for use where multiple devices may share a
single assigned MAC address and need to have different IPv6 addresses.
Siena's two ports each have their own MAC addresses.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>