media: xilinx: Refactor struct xvip_dma

Replace a single element array with a single field.

The following cocci warning is fixed:
drivers/media/platform/xilinx/xilinx-dma.h:100:19-22: WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

)

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>