[libata] sata_sis: use correct S/G table size

sata_sis has the same restrictions as other SFF controllers, and so must
use LIBATA_MAX_PRD to denote that SCSI may only fill ATA_MAX_PRD/2
entries, due to our need to handle IOMMU merging.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>