Blackfin arch: fix a compiling warning about dma-mapping

Signed-off-by: Bryan Wu <bryan.wu@analog.com>

diff --git a/arch/blackfin/kernel/dma-mapping.c b/arch/blackfin/kernel/dma-mapping.c
index ea48d5b..94d7b11 100644
--- a/arch/blackfin/kernel/dma-mapping.c
+++ b/arch/blackfin/kernel/dma-mapping.c
@@ -160,7 +160,8 @@
 	BUG_ON(direction == DMA_NONE);
 
 	for (i = 0; i < nents; i++, sg++) {
-		sg->dma_address = page_address(sg->page) + sg->offset;
+		sg->dma_address = (dma_addr_t)(page_address(sg->page) +
+					sg->offset);
 
 		invalidate_dcache_range(sg_dma_address(sg),
 					sg_dma_address(sg) +