scsi: hisi_sas: decrease running_req in hisi_sas_slot_task_free()

There is an issue that hisi_sas_dev.running_req is not
decremented properly for internal abort and TMF.

To resolve, only decrease running_req in hisi_sas_slot_task_free()

Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>