sl82c105: program DMA/PIO timings in ->dma_start/->ide_dma_end

* Program DMA timings in sl82c105_dma_start() (->dma_start method)
  before starting DMA transfer.

* Add sl82c105_dma_end() (->ide_dma_end method) to switch back to
  PIO timings when DMA transfer is complete.

* In sl82c105_set_pio_mode() program timings regardless of ->using_dma
  setting and in sl82c105_set_dma_mode() only cache the new timings.

* Remove no longer needed sl82c105_{ide_dma_on,off_quietly}().

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>