can: m_can: make custom bittiming fields const

The assigned timing structs will be defined a const anyway, so we can
avoid a few casts by declaring the struct fields as const as well.

Link: https://lore.kernel.org/all/4508fa4e639164b2584c49a065d90c78a91fa568.1636967198.git.matthias.schiffer@ew.tq-group.comSigned-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/m_can/m_can.h

@@ -85,8 +85,8 @@ struct m_can_classdev {
 	struct sk_buff *tx_skb;
 	struct phy *transceiver;
 
-	struct can_bittiming_const *bit_timing;
-	struct can_bittiming_const *data_timing;
+	const struct can_bittiming_const *bit_timing;
+	const struct can_bittiming_const *data_timing;
 
 	struct m_can_ops *ops;