ide: remove ->dma_prdtable field from ide_hwif_t

* Use 'hwif->dma_base + {4,8}' instead of hwif->dma_prdtable in
  {ide,scc}_dma_setup().

* Remove no longer needed ->dma_prdtable field from ide_hwif_t.

While at it:

* Use ATA_DMA_TABLE_OFS define.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>