scsi: hisi_sas: Fix spin lock management in slot_index_alloc_quirk_v2_hw()

Currently a spin_unlock_irqrestore() call is missing on the error path,
so add it.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>