pata_pdc202xx_old MWDMA fixes, and notes

I've been doing an audit of this driver to try and find out why we have
problems with some Clevo boxes that use it. Didn't get anywhere other
than to discover all the bug reporters I have use vmware, which may or
may not be chance.

In the process however I did find out our MWDMA2 performance was a bit
low and code review showed the MWDMA0/2 timings are reversed due to a
thinko in the table ordering

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>