ide-pmac: remove pmac_ide_{m,u}dma_enable() (take 2)

* Fix pmac_ide_dma_check() to use pmac_ide_tune_chipset() and remove no longer
  necessary pmac_ide_{m,u}dma_enable().

* While at it remove some dead code from pmac_ide_dma_check() (leftovers from
  conversion to use ide_max_dma_mode()).

There should be no functionality changes caused by this patch.

v2:
* Fix compile by replacing "id" with "drive->id" in pmac_ide_dma_check()
  (Noticed by Ben).

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