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: 8b4673ba ("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>