scsi: hisi_sas: fix return value of hisi_sas_task_prep()

It is an implicit regulation that error code that function returned should
be negative. But hisi_sas_task_prep() doesn't follow this.  This may cause
problems in the upper layer code.

For example, in sas_expander.c of libsas, smp_execute_task_sg() may return
the number of bytes of underrun. It will be conflicted with the scenaio
lldd_execute_task() return an positive error code.

This patch change the return value from SAS_PHY_DOWN to -ECOMM in
hisi_sas_task_prep().
Signed-off-by: NXiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: NJohn Garry <john.garry@huawei.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/hisi_sas/hisi_sas_main.c

@@ -316,7 +316,7 @@ static int hisi_sas_task_prep(struct sas_task *task, struct hisi_sas_dq
 		 */
 		if (device->dev_type != SAS_SATA_DEV)
 			task->task_done(task);
-		return SAS_PHY_DOWN;
+		return -ECOMM;
 	}
 
 	if (DEV_IS_GONE(sas_dev)) {
@@ -327,7 +327,7 @@ static int hisi_sas_task_prep(struct sas_task *task, struct hisi_sas_dq
 			dev_info(dev, "task prep: device %016llx not ready\n",
 				 SAS_ADDR(device->sas_addr));
 
-		return SAS_PHY_DOWN;
+		return -ECOMM;
 	}
 
 	port = to_hisi_sas_port(sas_port);
@@ -337,7 +337,7 @@ static int hisi_sas_task_prep(struct sas_task *task, struct hisi_sas_dq
 			 "SATA/STP" : "SAS",
 			 device->port->id);
 
-		return SAS_PHY_DOWN;
+		return -ECOMM;
 	}
 
 	if (!sas_protocol_ata(task->task_proto)) {