can: peak_usb: export PCAN CAN channel ID as sysfs device attribute

This patch exports the CAN channel ID as a sysfs attribute. The CAN
channel ID is a user-configurable u8/u32 identifier that can be set
individually for each CAN interface of a PEAK USB device.

Exporting the channel ID as a sysfs attribute allows users to easily read
the ID and to write udev rules that can match against the ID. This is
especially useful for PEAK USB devices that do not export a serial
number at SUB level.

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/20230116200932.157769-7-lukas.magel@posteo.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>