mtd: nand: fix scan_read_raw_oob

It seems there is a bug in scan_read_raw_oob() in nand_bbt.c which
should cause wrong functioning of NAND_BBT_SCANALLPAGES option.

Artem: the patch did not apply and I had to amend it a bit.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: stable@kernel.org
diff --git a/drivers/mtd/nand/nand_bbt.c b/drivers/mtd/nand/nand_bbt.c
index 20a112f..30d1319 100644
--- a/drivers/mtd/nand/nand_bbt.c
+++ b/drivers/mtd/nand/nand_bbt.c
@@ -324,6 +324,7 @@
 
 		buf += mtd->oobsize + mtd->writesize;
 		len -= mtd->writesize;
+		offs += mtd->writesize;
 	}
 	return 0;
 }