pata_it8213: fix wrong MWDMA timings being programmed

Clear old MWDMA timings before programming new ones (IT8213
is a single port host so there is no need to check ap->port_no).

This change 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>