can: peak_usb: correction of an initially misnamed field name

The data structure returned from the USB device contains a number
flashed by the user and not the serial number of the device.

Link: https://lore.kernel.org/all/20220719120632.26774-2-s.grosjean@peak-system.com


Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>