can: esd_usb2: Fix can_dlc value for received RTR, frames

The dlc member of the struct rx_msg contains also the ESD_RTR flag to
mark received RTR frames. Without the fix the can_dlc value for received
RTR frames would always be set to 8 by get_can_dlc() instead of the
received value.

Fixes: 96d8e903 ("can: Add driver for esd CAN-USB/2 device")
Signed-off-by: Stefan Mätje <stefan.maetje@esd.eu>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>