hisi_sas: fix error codes in hisi_sas_task_prep()

There were a couple cases where the error codes weren't set and also I
changed the success return to "return 0;" which is the same as
"return rc;" but more explicit.

Fixes: 42e7a693 ('hisi_sas: Add ssp command functio')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>