microblaze: Do not setup empty unmap_sg function

No reason to setup empty function. Core is checking
if this function should be called or not.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/kernel/dma.c

@@ -68,12 +68,6 @@ static int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl,
 	return nents;
 }
 
-static void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sg,
-				int nents, enum dma_data_direction direction,
-				struct dma_attrs *attrs)
-{
-}
-
 static int dma_direct_dma_supported(struct device *dev, u64 mask)
 {
 	return 1;
@@ -164,7 +158,6 @@ struct dma_map_ops dma_direct_ops = {
 	.alloc		= dma_direct_alloc_coherent,
 	.free		= dma_direct_free_coherent,
 	.map_sg		= dma_direct_map_sg,
-	.unmap_sg	= dma_direct_unmap_sg,
 	.dma_supported	= dma_direct_dma_supported,
 	.map_page	= dma_direct_map_page,
 	.unmap_page	= dma_direct_unmap_page,