scsi: hisi_sas: fail tmf task prep when port detached

When the port is detached we cannot execute a TMF, as there can be no
device attached to the port.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>