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/mtdpart.c b/drivers/mtd/mtdpart.c
index a5e7a21..d65af37 100644
--- a/drivers/mtd/mtdpart.c
+++ b/drivers/mtd/mtdpart.c
@@ -287,7 +287,7 @@
 	struct mtd_part *part = PART(mtd);
 	if ((len + ofs) > mtd->size)
 		return -EINVAL;
-	return part->master->unlock(part->master, ofs + part->offset, len);
+	return mtd_unlock(part->master, ofs + part->offset, len);
 }
 
 static int part_is_locked(struct mtd_info *mtd, loff_t ofs, uint64_t len)