mtd: introduce mtd_unlock 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 97d6360..272ebc0 100644
--- a/drivers/mtd/mtdconcat.c
+++ b/drivers/mtd/mtdconcat.c
@@ -596,7 +596,7 @@
 			size = len;
 
 		if (subdev->unlock) {
-			err = subdev->unlock(subdev, ofs, size);
+			err = mtd_unlock(subdev, ofs, size);
 			if (err)
 				break;
 		} else