media: vb2 dma-sg: Constify dma_buf_ops structures

dma_buf_ops are not supposed to change at runtime. All functions
working with dma_buf_ops provided by <linux/dma-buf.h> work with
const dma_buf_ops. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   5238	    112	      4	   5354	   14ea drivers/media/v4l2-core/videobuf2-dma-sg.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   5358	      0	      4	   5362	   14f2 drivers/media/v4l2-core/videobuf2-dma-sg.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/v4l2-core/videobuf2-dma-sg.c

@@ -500,7 +500,7 @@ static int vb2_dma_sg_dmabuf_ops_mmap(struct dma_buf *dbuf,
 	return vb2_dma_sg_mmap(dbuf->priv, vma);
 }
 
-static struct dma_buf_ops vb2_dma_sg_dmabuf_ops = {
+static const struct dma_buf_ops vb2_dma_sg_dmabuf_ops = {
 	.attach = vb2_dma_sg_dmabuf_ops_attach,
 	.detach = vb2_dma_sg_dmabuf_ops_detach,
 	.map_dma_buf = vb2_dma_sg_dmabuf_ops_map,