Use ->set_pio_mode method to program PIO modes in ide_set_xfer_rate()
(the only place which used ->speedproc to program PIO modes) and remove
handling of PIO modes from all ->speedproc implementations.

v2:
* Fix pmac_ide_tune_chipset() comment.

There should be no functionality changes caused by this patch.
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>