pata_efar: fix wrong MWDMA timings being programmed

Do not clear MWDMA timings for device on the other port when
programming slave device.

This change should be safe as this is how we have been doing
things in IDE slc90e66 host driver for years.

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