scsi: dma-mapping: always provide dma_get_cache_alignment

Provide the dummy version of dma_get_cache_alignment that always returns
1 even if CONFIG_HAS_DMA is not set, so that drivers and subsystems can
use it without ifdefs.

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