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

drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c

@@ -837,7 +837,7 @@ static int __mcp251xfd_get_berr_counter(const struct net_device *ndev,
 		return err;
 
 	if (trec & MCP251XFD_REG_TREC_TXBO)
-		bec->txerr = 256;
+		bec->txerr = CAN_BUS_OFF_THRESHOLD;
 	else
 		bec->txerr = FIELD_GET(MCP251XFD_REG_TREC_TEC_MASK, trec);
 	bec->rxerr = FIELD_GET(MCP251XFD_REG_TREC_REC_MASK, trec);