can: peak_usb: pcan_usb_encode_msg(): add information

This patch adds information by replacing a hard-coded values with its
symbol and a adding comment.

Link: https://lore.kernel.org/r/20210625130931.27438-5-s.grosjean@peak-system.com


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