can: m_can: update link to M_CAN user manual

Old versions of the user manual are regularly depublished, so change link to
the linux-can github page, which has a mirror off all published datasheets.

Link: https://lore.kernel.org/r/20201212175518.139651-2-mkl@pengutronix.de


Reviewed-by: Sean Nyekjaer <sean@geanix.com>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>