ide-pmac: fix set_timings_mdma()

* Move adjusting of cycle time for devices providing explicit DMA cycle time
  from pmac_ide_mdma_enable() to set_timings_mdma().

* Remove no longer needed drive_cycle_time argument.

* BUG() if unsupported speed argument value is passed (shouldn't happen).

* Matching access/recovery timings always exist so remove redundant check.

* Make set_timings_mdma() void.

* Update pmac_ide_tune_chipset()'s comment.

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