can: mcp251xfd: mcp251xfd_regmap_crc_read_one(): Factor out crc check into separate function

This patch factors out the crc check into a separate function. This is
preparation for the next patch.

Link: https://lore.kernel.org/r/20210406110617.1865592-4-mkl@pengutronix.de
Cc: Manivannan Sadhasivam <mani@kernel.org>
Cc: Thomas Kopp <thomas.kopp@microchip.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>