scsi: hisi_sas: Fix to only call scsi_get_prot_op() for non-NULL scsi_cmnd

A NULL-pointer dereference was introduced for TMF SSP commands from the
upstreaming reworking.

Fix this by relocating the scsi_get_prot_op() callsite.

Fixes: d6a9000b

 ("scsi: hisi_sas: Add support for DIF feature for v2 hw")
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>