can: peak_usb: pcan_usb_pro_encode_msg(): use macros for flags instead of plain integers

This patch replaces the plain integers used for flags in
pcan_usb_pro_encode_msg() by macros which are already defined.

Link: https://lore.kernel.org/r/20210309082128.23125-4-s.grosjean@peak-system.com
Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
[mkl: split into two patches]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>