can: mcp251xfd: __mcp251xfd_chip_set_mode(): prepare for PLL support: improve error handling and diagnostics

This patch prepares the __mcp251xfd_chip_set_mode() function for PLL
support by adding more error checks and diagnostics.

Link: https://lore.kernel.org/all/20220207131047.282110-13-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>