When sas_port is NULL, then return SAS_PHY_DOWN.

In addition, when the sas_dev is gone then explicitly return
SAS_PHY_DOWN.
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>