[ARM] Fix oops in dma_unmap_single()

Fix an oops caused by a missing failure return value for devices which
do not require DMA bouncing.

arch/arm/common/dmabounce.c

@@ -100,6 +100,7 @@ find_dmabounce_dev(struct device *dev)
 		if (d->dev == dev)
 			return d;
 	}
+	return NULL;
 }