can: mcp251xfd: mcp251xfd_tx_obj_from_skb(): don't copy data for RTR CAN frames in TX-path

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 optimizes the TX-path to not copy any data for RTR frames.

Link: https://lore.kernel.org/r/20210114153448.1506901-5-mkl@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>