can: m_can: use bits.h macros for all regmasks

This updates m_can.c to exclusively use GENMASK, FIELD_GET, FIELD_PREP
and FIELD_MAX for regmask ops, as is convention in the current kernel
(far less error-prone, far more concise).

Link: https://lore.kernel.org/r/20210504125123.500553-2-torin@maxiluxsystems.comSigned-off-by: Torin Cooper-Bennun <torin@maxiluxsystems.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>