ide-pmac: use ide_tune_dma() (take 2)

* Add missing initialization of hwif->autodma and drive->autodma to
  pmac_ide_setup_dma().

* Use ide_tune_dma() in pmac_ide_dma_check().

While at it:

* Fix pmac_ide_dma_check() return value if DMA mode is not programmed
  (should be "-1" otherwise ide_set_dma() will try to enable DMA).

* Remove unnecessary drive->using_dma fiddling (->dma_off_quietly is always
  called before ide_set_dma() call and ide_set_dma() calls ->ide_dma_on if
  ->ide_dma_check returns "0").

v2:
* No reason to blacklist all ide_floppy devices and the old code was always
  enabling DMA anyway (without even programming controller/device if the
  device was ide_floppy).

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>