I think that I've found and fixed the problem. There is a copy/paste bug in
vt6421_set_dma_mode() function which causes wrong values to be written to
PATA_UDMA_TIMING register.

This patch fixes a copy/paste bug that breaks DMA modes on VT6421 PATA port.
Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>