can: mcp251xfd: mcp251xfd_chip_sleep(): introduce function to bring chip into sleep mode

This patch adds a new function to bring the chip into sleep mode, and
replaces several occurrences of open coded variants.

Link: https://lore.kernel.org/all/20220207131047.282110-5-mkl@pengutronix.de


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