[SCSI] isci: fix needless ata reset escalations

isci is needlessly tying libata's hands by returning
SAM_STAT_CHECK_CONDITION to some ata errors.  Instead, prefer
SAS_PROTO_RESPONSE to let libata (via sas_ata_task_done()) disposition
the device-to-host fis.

For example isci is triggering an HSM Violation where AHCI is showing a
simple media error for the same bus condition:

isci:
ata7.00: failed command: READ VERIFY SECTOR(S)
ata7.00: cmd 40/00:01:00:00:00/00:00:00:00:00/e0 tag 0
         res 01/04:00:00:00:00/00:00:00:00:00/e0 Emask 0x3 (HSM violation)

ahci:
ata6.00: failed command: READ VERIFY SECTOR(S)
ata6.00: cmd 40/00:01:00:00:00/00:00:00:00:00/e0 tag 0
         res 51/40:01:00:00:00/00:00:00:00:00/e0 Emask 0x9 (media error)

Note that the isci response matches this from sas_ata_task_done():
	/* We saw a SAS error. Send a vague error. */
	[..]
	dev->sata_dev.fis[3] = 0x04; /* status err */
	dev->sata_dev.fis[2] = ATA_ERR;

The end effect is that isci is needlessly triggering hard resets when
they are not necessary.
Reported-by: NXun Ni <xun.ni@intel.com>
Tested-by: NNelson Cheng <nelson.cheng@intel.com>
Acked-by: NLukasz Dorau <lukasz.dorau@intel.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>

drivers/scsi/isci/request.c

@@ -2723,13 +2723,9 @@ static void isci_process_stp_response(struct sas_task *task, struct dev_to_host_
 	memcpy(resp->ending_fis, fis, sizeof(*fis));
 	ts->buf_valid_size = sizeof(*resp);
 
-	/* If the device fault bit is set in the status register, then
-	 * set the sense data and return.
-	 */
-	if (fis->status & ATA_DF)
+	/* If an error is flagged let libata decode the fis */
+	if (ac_err_mask(fis->status))
 		ts->stat = SAS_PROTO_RESPONSE;
-	else if (fis->status & ATA_ERR)
-		ts->stat = SAM_STAT_CHECK_CONDITION;
 	else
 		ts->stat = SAM_STAT_GOOD;