can: mcp251xfd: replace sizeof(u32) with val_bytes in regmap

The sizeof(u32) is hardcoded. It's better to use the config value from the
regmap.

It increases the size of target object, but it's flexible when new mcp chip
need other val_bytes.

Link: https://lore.kernel.org/r/20210122081334.213957-1-suyanjun218@gmail.com


Signed-off-by: Su Yanjun <suyanjun218@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>