can: etas_es58x: es58x_init_netdev: populate net_device::dev_port

The field dev_port of struct net_device indicates the port number of a
network device [1]. This patch populates this field.

This field can be helpful to distinguish between the two network
interfaces of a dual channel device (i.e. ES581.4 or ES582.1). Indeed,
at the moment, all the network interfaces of a same device share the
same static udev attributes c.f. output of:

| udevadm info --attribute-walk /sys/class/net/canX

The dev_port attribute can then be used to write some udev rules to,
for example, assign a permanent name to each network interface based
on the serial/dev_port pair (which is convenient when you have a test
bench with several CAN devices connected simultaneously and wish to
keep consistent interface names upon reboot).

[1] https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-class-net

Link: https://lore.kernel.org/all/20211026180553.1953189-1-mailhol.vincent@wanadoo.frSuggested-by: Lukas Magel <lukas.magel@escrypt.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/etas_es58x/es58x_core.c

@@ -2094,6 +2094,7 @@ static int es58x_init_netdev(struct es58x_device *es58x_dev, int channel_idx)
 
 	netdev->netdev_ops = &es58x_netdev_ops;
 	netdev->flags |= IFF_ECHO;	/* We support local echo */
+	netdev->dev_port = channel_idx;
 
 	ret = register_candev(netdev);
 	if (ret)