SAS drivers can be compiled with ata support disabled.  Provide a stub so
that the drivers don't have to ifdef around wiring up
ata_scsi_dma_need_drain.

Link: https://lore.kernel.org/r/20200615064624.37317-2-hch@lst.deSigned-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>