pata_sis: FIFO whack

If you are using a SiS controller and the BIOS didn't set it up then the
FIFO may be left active when we try and set up the CD. Not convinced this
matters but I'd prefer to be safe

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