can: peak_usb: PCAN-USB specific part: fix little endian usage

That patch fixes some bad usage of two little-endian variables, which lead to
some warning/error when building the peak_usb driver.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>