pata_it8213: fix wrong PIO timings being programmed

* do not clear PIO timings for master when programming slave
* program new PIO timings in the correct register nibble

Both changes should be safe as this is how we have been doing
things in IDE it8213 host driver for years.

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