scsi: hisi_sas: Put reserved tags in lower region of tagset

To be consistent with blk-mq, put the reserved tags in the lower region of
the tagset. Eventually we hope to get rid of all this reserved tag
management.

Signed-off-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/1666091763-11023-4-git-send-email-john.garry@huawei.com
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>