mtd: introduce mtd_write interface

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/drivers/mtd/mtdconcat.c b/drivers/mtd/mtdconcat.c
index 4546034..4521550 100644
--- a/drivers/mtd/mtdconcat.c
+++ b/drivers/mtd/mtdconcat.c
@@ -148,7 +148,7 @@
 		if (!(subdev->flags & MTD_WRITEABLE))
 			err = -EROFS;
 		else
-			err = subdev->write(subdev, to, size, &retsize, buf);
+			err = mtd_write(subdev, to, size, &retsize, buf);
 
 		if (err)
 			break;