In Classical CAN there are RTR frames. RTR frames have the RTR bit set, may
have a dlc != 0, but contain no data.

This patch changes the RX-path to no copy any data for RTR frames, so that the
data field in the CAN frame stays 0x0.

Link: https://lore.kernel.org/r/20210114153448.1506901-4-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>