pata_hpt*, pata_serverworks: fix UDMA masking

When masking, mask out the modes that are unsupported not the ones
that are supported.  This makes life happier.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>