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>