scsi: qla2xxx: don't allow negative thresholds

We shouldn't allow negative thresholds.  I don't know what it would do
but it can't be good.

Fixes: 8b4673ba3a ("scsi: qla2xxx: Add support for ZIO6 interrupt threshold")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/qla2xxx/qla_attr.c

@@ -1228,7 +1228,7 @@ qla_zio_threshold_store(struct device *dev, struct device_attribute *attr,
 		return -EINVAL;
 	if (sscanf(buf, "%d", &val) != 1)
 		return -EINVAL;
-	if (val > 256)
+	if (val < 0 || val > 256)
 		return -ERANGE;
 
 	atomic_set(&vha->hw->zio_threshold, val);