mac_dbdma: always initialize channel field in DBDMA_channel

dbdma_from_ch() uses channel field to return the right DBDMA object.
Previous code was working if guest OS was only using registered DMA channels.
However, it lead to QEMU crashes if guest OS was using unregistered DMA channels.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
