IB/ipoib: Use dev_port to expose network interface port numbers

Some InfiniBand network devices have multiple ports on the same PCI
function. This initializes the `dev_port' sysfs field of those
network interfaces with their port number.

Prior to this the kernel erroneously used the `dev_id' sysfs
field of those network interfaces to convey the port number to userspace.

The use of `dev_id' was considered correct until Linux 3.15,
when another field, `dev_port', was defined for this particular
purpose and `dev_id' was reserved for distinguishing stacked ifaces
(e.g: VLANs) with the same hardware address as their parent device.

Similar fixes to net/mlx4_en and many other drivers, which started
exporting this information through `dev_id' before 3.15, were accepted
into the kernel 4 years ago.
See 76a066f2

 (`net/mlx4_en: Expose port number through sysfs').

Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
Signed-off-by: Doug Ledford <dledford@redhat.com>