pdc202xx_new: use ide_tune_dma()

* remove code enabling IORDY and prefetch from config_chipset_for_dma(),
  as the comment states it has no real effect because these settings are
  overriden when the PIO mode is set (and for this driver ->autotune == 1
  so PIO mode is always programmed)

* use ide_tune_dma() in pdcnew_config_drive_xfer_rate() and remove no longer
  needed config_chipset_for_dma()

There should be no functionality changes caused by this patch.

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