Presently any received buffers are only released back to vb2 if
vsp1_video_stop_streaming() is called. If vsp1_video_start_streaming()
encounters an error, we will be warned by the vb2 handlers that buffers
have not been returned.

Move the buffer cleanup code to its own function to prevent duplication
and call from both vsp1_video_stop_streaming() and the error path in
vsp1_video_start_streaming().
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>