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/mtdconcat.c b/drivers/mtd/mtdconcat.c
index 4b7f825..d0db5e6 100644
--- a/drivers/mtd/mtdconcat.c
+++ b/drivers/mtd/mtdconcat.c
@@ -667,7 +667,7 @@
 			continue;
 		}
 
-		res = subdev->block_isbad(subdev, ofs);
+		res = mtd_block_isbad(subdev, ofs);
 		break;
 	}