can: xilinx_can: fix the data update logic for CANFD FD frames

commit c223da68 ("can: xilinx_can: Add support for CANFD FD frames")
is writing data to a wrong offset for FD frames.

This patch fixes this issue.

Fixes: c223da68

 ("can: xilinx_can: Add support for CANFD FD frames")
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Reviewed-by: Shubhrajyoti Datta <Shubhrajyoti.datta@xilinx.com>
Signed-off-by: Appana Durga Kedareswara rao <appana.durga.rao@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>