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/mcp251x.c b/drivers/net/can/mcp251x.c
index 9120a36..a580db2 100644
--- a/drivers/net/can/mcp251x.c
+++ b/drivers/net/can/mcp251x.c
@@ -214,7 +214,7 @@
 module_param(mcp251x_enable_dma, int, S_IRUGO);
 MODULE_PARM_DESC(mcp251x_enable_dma, "Enable SPI DMA. Default: 0 (Off)");
 
-static struct can_bittiming_const mcp251x_bittiming_const = {
+static const struct can_bittiming_const mcp251x_bittiming_const = {
 	.name = DEVICE_NAME,
 	.tseg1_min = 3,
 	.tseg1_max = 16,