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/drivers/net/can/pch_can.c b/drivers/net/can/pch_can.c
index 1226297..48b3d62 100644
--- a/drivers/net/can/pch_can.c
+++ b/drivers/net/can/pch_can.c
@@ -184,7 +184,7 @@
 	int use_msi;
 };
 
-static struct can_bittiming_const pch_can_bittiming_const = {
+static const struct can_bittiming_const pch_can_bittiming_const = {
 	.name = KBUILD_MODNAME,
 	.tseg1_min = 2,
 	.tseg1_max = 16,