Factor out the functionality of assigning a SJW default value into
can_sjw_set_default() and the checking the SJW limits into
can_sjw_check().

This functions will be improved and called from a different function
in the following patches.

Link: https://lore.kernel.org/all/20230202110854.2318594-11-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>