ide-pmac: skip conservative PIO "downgrade"

We can skip conservative PIO "downgrade" (PIO3 becomes PIO2 etc.) on PMAC.

Problem reported by Mikael.

Cc: Mikael Pettersson <mikpe@it.uu.se>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>