can: mcp251x: fix reception of standard RTR frames

This patch sets the missing CAN_RTR_FLAG if a standard RTR frame is
received.
(For extended frames the CAN_RTR_FLAG was already set correctly.)

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>