This completely reworks the handling of the control register
according to my understanding of the HW and the spec.

It should (hopefully ... still testing) fix a number of issues
most notably cases of MacOS hanging.

Also update dbdma_unassigned_rw() and dbdma_unassigned_flush() to
have the expected behaviour now that flush is handled slightly
differently.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>