pata_cmd64x: Set up MWDMA modes properly

Set the MWDMA timing by updating the correct registers.  Split the PIO path as
this is mostly shared code.  Wants testing.

Signed-off-by: Alan Cox <alan@redhat.com>
Tested-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>