ata_piix: unify code for programming PIO and MWDMA timings

Besides making things noticably simpler it results in ~2% decrease in
the driver LOC count and also ~2% decrease in the driver binary size
(as measured on x86-32).

Fix piix_set_piomode() documentation while at it.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>