dbdma: reset io->processing flag for unassigned DBDMA channel rw accesses

Otherwise MacOS 9 hangs upon shutdown.

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