mtd: introduce mtd_read_oob 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 4521550..cf35642 100644
--- a/drivers/mtd/mtdconcat.c
+++ b/drivers/mtd/mtdconcat.c
@@ -273,7 +273,7 @@
 		if (from + devops.len > subdev->size)
 			devops.len = subdev->size - from;
 
-		err = subdev->read_oob(subdev, from, &devops);
+		err = mtd_read_oob(subdev, from, &devops);
 		ops->retlen += devops.retlen;
 		ops->oobretlen += devops.oobretlen;