NAND: Update read_read_subpage API check

This patch updates a check condition in the NAND driver.
The check condition is similat to what is in linux/next.

Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>

drivers/mtd/nand/nand_base.c

@@ -1061,7 +1061,7 @@ 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 < 0)
+		if (stat == -1)
 			mtd->ecc_stats.failed++;
 		else
 			mtd->ecc_stats.corrected += stat;