- keep the modes supported by the drive in struct ata_drive_datas (will be
later used for downgrading the DMA/PIO mode on error)
- use config flags to force/disable PIO/DMA/UDMA modes
- For the CMD PCI0643/6 setup DMA mode to DMA Read multiple.
device registers should be read back into the wdc_command structure after
successfull command completion. Use this this in wdioctl() for
ATAIOCCOMMAND.