ide-pmac: pmac_ide_tune_chipset() fixes

* Don't check check for pmif == NULL (it should never be NULL if we got here).

* Make a local copy of the timings and set the pmif->timings[] only after
  setting the transfer mode on the device (otherwise SELECT_DRIVE() call in
  pmac_ide_do_setfeature() will program new timings before the transfer mode
  is set on the device - this was pointed out by Sergei).  This change makes
  pmac_ide_tune_chipset() behavior match this of pmac_ide_{m,u}dma_enable().

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