[libata] MWDMA0 is unsupported on PIIX-like PATA controllers

MWDMA0 timings cannot be met with the PIIX based controller
programming interface.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>