piix: cleanup

* disable DMA masks if no_piix_dma is set and remove now
  not needed no_piix_dma_check from piix_config_drive_for_dma()
* there is no need to read register 0x55 in init_hwif_piix()
* move cable detection code to piix_cable_detect()
* remove unreachable 82371MX code from init_hwif_piix()

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