scsi: Add NO_DMA dummies for SCSI DMA mapping API

Add dummies for scsi_dma_{,un}map(), to allow compile-testing if
NO_DMA=y.

This prevents the following from showing up later:

    ERROR: "scsi_dma_unmap" [drivers/firewire/firewire-sbp2.ko] undefined!
    ERROR: "scsi_dma_map" [drivers/firewire/firewire-sbp2.ko] undefined!

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>