mtd: introduce mtd_sync interface

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/ubi/kapi.c

@@ -715,7 +715,7 @@ int ubi_sync(int ubi_num)
 		return -ENODEV;
 
 	if (ubi->mtd->sync)
-		ubi->mtd->sync(ubi->mtd);
+		mtd_sync(ubi->mtd);
 
 	ubi_put_device(ubi);
 	return 0;