pata_radisys: fix mwdma_mask to exclude mwdma0

As noted by Alan:
>Your suspicions are correct here btw - the device can only do MWDMA1 and
>MWDMA2 (much like some PIIX devices)

Signed-off-by: Erik Inge Bolsø <knan-lkml@anduin.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>