scsi: hisi_sas: Stop using queue #0 always for v2 hw

In commit 8d98416a ("scsi: hisi_sas: Switch v3 hw to MQ"), the dispatch
function was changed to choose the delivery queue based on the request tag
HW queue index.

This heavily degrades performance for v2 hw, since the HW queues are not
exposed there, and, as such, HW queue #0 is used for every command.

Revert to previous behaviour for when nr_hw_queues is not set, that being
to choose the HW queue based on target device index.

Link: https://lore.kernel.org/r/1602750425-240341-1-git-send-email-john.garry@huawei.com
Fixes: 8d98416a ("scsi: hisi_sas: Switch v3 hw to MQ")
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/hisi_sas/hisi_sas_main.c

@@ -445,7 +445,7 @@ static int hisi_sas_task_prep(struct sas_task *task,
 		}
 	}
 
-	if (scmd) {
+	if (scmd && hisi_hba->shost->nr_hw_queues) {
 		unsigned int dq_index;
 		u32 blk_tag;