scsi: hisi_sas: fix a bug in hisi_sas_dev_gone()

scsi: hisi_sas: fix a bug in hisi_sas_dev_gone()

When device gone, NULL pointer can be accessed in free_device callback
if during SAS controller reset as we clear structure sas_dev prior.

Actually we can only set dev_type as SAS_PHY_UNUSED and not clear
structure sas_dev as all the members of structure sas_dev will be
re-initialized after device found.

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>