mtd: onenand: fix warning (integer used as pointer)

Fixes this sparse warning:

    CHECK   drivers/mtd/onenand/generic.c
  drivers/mtd/onenand/generic.c:61:62: warning: Using plain integer as NULL pointer
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/onenand/generic.c

@@ -58,7 +58,7 @@ static int generic_onenand_probe(struct platform_device *pdev)
 		goto out_release_mem_region;
 	}
 
-	info->onenand.mmcontrol = pdata ? pdata->mmcontrol : 0;
+	info->onenand.mmcontrol = pdata ? pdata->mmcontrol : NULL;
 	info->onenand.irq = platform_get_irq(pdev, 0);
 
 	info->mtd.name = dev_name(&pdev->dev);