mtd: rawnand: fix return value check for bad block status

Positive return value from read_oob() is making false BAD
blocks. For some of the NAND controllers, OOB bytes will be
protected with ECC and read_oob() will return number of bitflips.
If there is any bitflip in ECC protected OOB bytes for BAD block
status page, then that block is getting treated as BAD.

Fixes: c120e75e0e7d ("mtd: nand: use read_oob() instead of cmdfunc() for bad block check")
Cc: <stable@vger.kernel.org>
Signed-off-by: Abhishek Sahu <absahu@codeaurora.org>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>

drivers/mtd/nand/raw/nand_base.c

@@ -440,7 +440,7 @@ static int nand_block_bad(struct mtd_info *mtd, loff_t ofs)
 
 	for (; page < page_end; page++) {
 		res = chip->ecc.read_oob(mtd, chip, page);
-		if (res)
+		if (res < 0)
 			return res;
 
 		bad = chip->oob_poi[chip->badblockpos];