can: mcp25xxfd: add listen-only mode

This commit enables listen-only mode, which works internally like CANFD mode.

Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20200918172536.2074504-5-mkl@pengutronix.de