scsi: scsi_transport_sas: switch to SPDX tags

Use the the GPLv2 SPDX tag instead of a free form blurb.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>