ide-pmac: PIO mode setup fixes (take 3)

* Add { 0, 0 } entry to {kauai,shasta}_pio_timings[] so kauai_lookup_timing()
  always returns a valid PIO timing (fixes PIO timing not being set for devices
  with minimum PIO cycle <= 120ns).

* Add setting transfer mode on the device to pmac_ide_set_pio_mode().

* Fix pmac_ide_set_pio() to always program chipset for given PIO timing instead
  of only when the device we want to program PIO timing for is the currently
  selected one.

* Now that pmac_ide_set_pio() is fixed there is no need to set transfer mode
  on the device and program chipset for PIO in pmac_ide_tune_chipset()
  (returning 0 == success is not entirely correct but is OK for now since
  the upper layers are only checking ->speedproc return value for DMA modes).

v2:
* s/speed/XFER_PIO_0 + pio/ in pmac_ide_set_pio_mode() so it actually compiles.

* Removal of kauai_lookup_timing() return value checking went to separate patch.

v3:
* Use XFER_PIO_0 not XFER_PIO. (Noticed by Ben)

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