can: mcp251xfd: __mcp251xfd_get_berr_counter(): use CAN_BUS_OFF_THRESHOLD instead of open coding it

Since 3f9c2621

 ("can: error: add definitions for the different CAN
error thresholds") we have proper defines for the various CAN error
thresholds. So make use of it and replace 256 by
CAN_BUS_OFF_THRESHOLD.

Link: https://lore.kernel.org/all/20240304074503.3584662-1-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>