can: m_can: fix whitespace in a few comments

Fixes whitespace in comments titling sections of register masks.

Link: https://lore.kernel.org/r/20210504125123.500553-5-torin@maxiluxsystems.com
Signed-off-by: Torin Cooper-Bennun <torin@maxiluxsystems.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>