scsi: hisi_sas: Use device lock to protect slot alloc/free

The IPTT of a slot is unique, and we currently use hisi_hba lock to
protect it.

Now slot is managed on hisi_sas_device.list, so use DQ lock to protect
for allocating and freeing the slot.

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>