can: mcp251xfd: add len8_dlc support

This patch adds support for the Classical CAN raw DLC functionality to send and
receive DLC values from 9 ... 15 to the mcp251xfd driver.

Link: https://lore.kernel.org/r/20210114153448.1506901-6-mkl@pengutronix.de


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