mtd: fix callback return value check

Drivers may (and do) return negative errno values other than -1 from the
ecc.correct callback.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/nand/nand_base.c

@@ -1223,7 +1223,7 @@ static int nand_read_subpage(struct mtd_info *mtd, struct nand_chip *chip, uint3
 		int stat;
 
 		stat = chip->ecc.correct(mtd, p, &chip->buffers->ecccode[i], &chip->buffers->ecccalc[i]);
-		if (stat == -1)
+		if (stat < 0)
 			mtd->ecc_stats.failed++;
 		else
 			mtd->ecc_stats.corrected += stat;