scsi: hisi_sas: grab hisi_hba.lock when processing slots

When adding/removing slots from device list, we need to lock this
operation with hisi_hba lock for safety.

This patch adds missing instances of this.

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>