ide: ide_start_power_step() fix WRT disabling DMA

* Do the same thing as probe_hwif() and always disable DMA so chipset DMA
  enabled bit gets cleared (if the drive doesn't support DMA ide_set_dma()
  won't try to tune it anyway).

* Add TODO comment about respecting ->using_dma setting.

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