can: mcp251xfd: unify error messages and commets

This patch unifies the error messages:
- have a "." and the end of each message
- write controller with a small "c", if not the first word of an error
  message.

Link: https://lore.kernel.org/r/20210128104644.2982125-6-mkl@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>