pmac_ide_tune_chipset() don't set drive->init_speed.

Fix it by setting drive->{current,init}_speed in pmac_ide_do_setfeature()
and clean up pmac_ide_{tune_chipset,mdma_enable,udma_enable}().
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>