scsi: libata: Provide an ata_scsi_dma_need_drain stub for !CONFIG_ATA

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