scsi: hisi_sas: Fix return value when get_free_slot() failed

It is an step of executing task to get free slot. If the step fails, we
will cleanup LLDD resources and should return failure to upper layer or
internal caller to abort task execution of this time.

But in the current code, the caller of get_free_slot() doesn't return
failure when get_free_slot() failed. This patch is to fix it.

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>