scsi: hisi_sas: Fix the code logic issue of NULL pointer check about sas_port in func:hisi_sas_task_prep

sas_port is a pointer passed by device->port,it accept non-zero check
before running ahead to prevent a null pointer to be passed on.

but some logic issue exist here that sas_port(device->port) was used before
non-zero check,such as func: dev_to_hisi_hba lay on line 290

290 struct hisi_hba #hisi_hba = dev_to_hisi_hba(device)
                                    ^
The code of function:dev_to_hisi_hib is as blow:

return device->port->ha->lldd_ha;

Obviously,device->port is used before non-zero check for non-zero check
begin at line 303 but dev_to_hisi_hba lay on line 290;

so,to fix this bug,I need to shift some code and make sure sas_port
which was used after non-zero check.
Signed-off-by: Nluojiaxing <luojiaxing@huawei.com>
Signed-off-by: NJohn Garry <john.garry@huawei.com>