can: peak_usb: rename device_id to CAN channel ID

The so-called "device id" is a user-defined identifier value that can be
set individually for each CAN interface of a PEAK USB device.
Contrary to a static serial number, the value can be changed by the
user. With this ID, each CAN interface can be uniquely identified even if
the USB device does not export a proper serial number or the USB device
exports multiple CAN interfaces. In order to not confuse it with the
device ID used by the USB core and emphasize the link to the CAN
interface, the functions and variables for reading this user-defined
value are renamed to CAN channel ID.

Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Lukas Magel <lukas.magel@posteo.net>
Link: https://lore.kernel.org/all/20230116200932.157769-2-lukas.magel@posteo.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>