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

Otherwise MacOS 9 hangs upon shutdown.
Signed-off-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/misc/macio/mac_dbdma.c

@@ -778,6 +778,7 @@ static void dbdma_unassigned_rw(DBDMA_io *io)
     DBDMA_channel *ch = io->channel;
     qemu_log_mask(LOG_GUEST_ERROR, "%s: use of unassigned channel %d\n",
                   __func__, ch->channel);
+    ch->io.processing = false;
 }
 
 static void dbdma_unassigned_flush(DBDMA_io *io)