sgiioc4: use ->extra_base instead of ->dma_status for dma_handle

This is a preparation for removal of ->dma_status field from ide_hwif_t.

There should be no functional changes caused by this patch.

Acked-by: Jeremy Higdon <jeremy@sgi.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>