media: xilinx: dma: stop_streaming should return void

Fix our driver to follow change in mainline code.
Based on mainline patch:
"[media] vb2: stop_streaming should return void"
(sha1: e37559b2)

Warning log:
drivers/media/platform/xilinx/xilinx-dma.c:505:2: warning:
initialization from incompatible pointer type [enabled by default]
  .stop_streaming = xvip_dma_stop_streaming,
  ^
drivers/media/platform/xilinx/xilinx-dma.c:505:2: warning: (near
initialization for 'xvip_dma_queue_qops.stop_streaming') [enabled by
default]

Reported-by: Wendy Liang <wendy.liang@xilinx.com>
Acked-by: Christian Kohn <christian.kohn@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>