pata_serverworks: Fix problem with some drive combinations

The driver used the channel not the device number for deciding where to
load some timing parameters. Also change so that we clear the UDMA field
as the old driver did. Not believed neccessary but does no harm.

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