mtd: introduce mtd_block_isbad 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 6d598b23..a499bf7 100644
--- a/drivers/mtd/mtdchar.c
+++ b/drivers/mtd/mtdchar.c
@@ -886,7 +886,7 @@
 		if (!mtd->block_isbad)
 			ret = -EOPNOTSUPP;
 		else
-			return mtd->block_isbad(mtd, offs);
+			return mtd_block_isbad(mtd, offs);
 		break;
 	}