scsi: hisi_sas: Check sas_port before using it

Need to check the structure sas_port before using it.

Link: https://lore.kernel.org/r/1573551059-107873-2-git-send-email-john.garry@huawei.com


Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>