sc1200: fix ->dma_base equal zero handling

Set hwif->atapi_dma/{ultra,mwdma}_mask and drive->autodma after checking that
->dma_base exists.  If ->dma_base is not set (== PCI BAR4 cannot be reserved)
then DMA hooks shouldn't be initialized or bad things will happen.

OTOH hwif->set_{pio,dma}_mode hooks should be set even if hwif->dma_base == 0.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>