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

drivers/net/can/m_can/m_can.c

@@ -5,8 +5,7 @@
 // Copyright (C) 2018-19 Texas Instruments Incorporated - http://www.ti.com/
 
 /* Bosch M_CAN user manual can be obtained from:
- * http://www.bosch-semiconductors.de/media/pdf_1/ipmodules_1/m_can/
- * mcan_users_manual_v302.pdf
+ * https://github.com/linux-can/can-doc/tree/master/m_can
  */
 
 #include <linux/interrupt.h>