can: mark bittiming_const pointer in struct can_priv as const

This patch marks the bittiming_const pointer as in the struct can_pric as
"const". This allows us to mark the struct can_bittiming_const in the CAN
drivers as "const", too.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index ee5a771..2b2fc34 100644
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -33,7 +33,7 @@
 	struct can_device_stats can_stats;
 
 	struct can_bittiming bittiming;
-	struct can_bittiming_const *bittiming_const;
+	const struct can_bittiming_const *bittiming_const;
 	struct can_clock clock;
 
 	enum can_state state;