idpf: distinguish vports by the dev_port attribute

idpf registers multiple netdevs (virtual ports) for one PCI function,
but it does not provide a way for userspace to distinguish them with
sysfs attributes. Per Documentation/ABI/testing/sysfs-class-net, it is
a bug not to set dev_port for independent ports on the same PCI bus,
device and function.

Without dev_port set, systemd-udevd's default naming policy attempts
to assign the same name ("ens2f0") to all four idpf netdevs on my test
system and obviously fails, leaving three of them with the initial
eth<N> name.

With this patch, systemd-udevd is able to assign unique names to the
netdevs (e.g. "ens2f0", "ens2f0d1", "ens2f0d2", "ens2f0d3").

The Intel-provided out-of-tree idpf driver already sets dev_port. In
this patch I chose to do it in the same place in the idpf_cfg_netdev
function.

Fixes: 0fe45467 ("idpf: add create vport and netdev configuration")
Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>