can: peak_usb: add ethtool interface to user-configurable CAN channel identifier

This patch introduces 3 new functions implementing support for ethtool
access to the CAN channel ID of all USB CAN network interfaces managed by
the driver. With this patch, it is possible to read/write the CAN
channel ID from/to the EEPROM via the ethtool interface.

The CAN channel ID is a user-configurable device identifier that can be
set individually for each CAN interface of a PEAK USB device. Depending on
the device, the identifier has a length of 8 or 32 bit. The identifier
is stored in the non-volatile memory of the device.

The identifier of a CAN interface can be read/written as an 8 or 32 bit
byte string in native (little-endian) byte order, where the length depends
on the device type.

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/20230116200932.157769-6-lukas.magel@posteo.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>