pata_atp867x: fix it to not claim MWDMA support

MWDMA modes are not supported by this driver currently.

Acked-by: Jung-Ik (John) Lee <jilee@google.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>