can: pcan_usb: Document the commands sent to the device

This patch documents the functions and numbers of the commands sent by the
driver to the PCAN-USB CAN device from PEAK-System GmbH.

Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Link: https://lore.kernel.org/r/20191206153803.17725-2-s.grosjean@peak-system.com


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>