sfc: stop setting dev_port

Setting dev_port changes the device names allocated by systemd. Any devices
with a dev_port >0 will (in default distro configurations) have a suffix of
"d<port-number>" appended.

This is not something done by other drivers, and causes confusion for users.

Fixes: 8be41320 ("sfc: Add code to export port_num in netdev->dev_port")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>