can: mcp251x: fix generation of error frames

The function "mcp251x_error_skb" is used to generate error frames.
They are identified by the "CAN_ERR_FLAG" in can_id. The function
overwrites the can_id so that the frames show up as normal frames instead
of error frames.

This patch fixes the problem by or'ing the can_id instead of overwriting it.
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Tested-by: Jargalan Nermunkh <jargalan.nermunkh@criticallink.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/can/mcp251x.c

@@ -705,7 +705,7 @@ static void mcp251x_error_skb(struct net_device *net, int can_id, int data1)
 
 	skb = alloc_can_err_skb(net, &frame);
 	if (skb) {
-		frame->can_id = can_id;
+		frame->can_id |= can_id;
 		frame->data[1] = data1;
 		netif_rx_ni(skb);
 	} else {