macio: set res_count value to 0 after non-block ATAPI DMA transfers

res_count should be set to the number of outstanding bytes after a DBDMA
request. Unfortunately this wasn't being set to zero by the non-block
transfer codepath meaning drivers that checked the descriptor result for
such requests (e.g reading the CDROM TOC) would assume from a non-zero result
that the transfer had failed.
Signed-off-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ide/macio.c

@@ -273,6 +273,7 @@ static void pmac_ide_atapi_transfer_cb(void *opaque, int ret)
         s->io_buffer_size = MIN(s->io_buffer_size, io->len);
         dma_memory_write(&address_space_memory, io->addr, s->io_buffer,
                          s->io_buffer_size);
+        io->len = 0;
         ide_atapi_cmd_ok(s);
         m->dma_active = false;
         goto done;