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/mtdchar.c b/drivers/mtd/mtdchar.c
index 870f2cb..fe09cd2 100644
--- a/drivers/mtd/mtdchar.c
+++ b/drivers/mtd/mtdchar.c
@@ -838,7 +838,7 @@
 		if (!mtd->unlock)
 			ret = -EOPNOTSUPP;
 		else
-			ret = mtd->unlock(mtd, einfo.start, einfo.length);
+			ret = mtd_unlock(mtd, einfo.start, einfo.length);
 		break;
 	}