libata-sff; Unbreak non DMA capable controllers again

Seems nobody else is checking/testing this case as it keeps getting
horked.

If we have no BAR4 mapping on an SFF controller this is *NOT* an error,
it just means it isn't doing BMDMA.

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