scsi: sd: add a SPDX tag to sd.c

sd.c is the only sd file missing licensing information.  Add a
GPLv2 tag for the default kernel license.

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>