ata_piix: fix MWDMA handling on PIIX3

Fix erroneous check for ap->udma_mask in do_pata_set_dmamode()
resulting in controller not being programmed properly for MWDMA.

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