cs5530/sc1200: DMA support cleanup

sc1200.c:

* remove open-coded variant of ide_dma_host_off() (== ->dma_host_off),
  it is not needed because ->dma_off_quietly calls ->dma_host_off

* use ->dma_host_on (== ide_dma_host_on() for this driver) instead of
  open-coded variant, call it from the users of sc1200_config_dma2()

  [ there is no need to call ->dma_host_on in sc1200_config_dma() because
    core code takes care of calling ->ide_dma_on on successful execution
    of ->ide_dma_check ]

* add comment about ->tuneproc interface abuse

cs5530.c/sc1200.c:

* core code takes care of calling ->dma_off_quietly before calling
  ->ide_dma_check so there is no need to call it in ->ide_dma_check methods

* bump driver version

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