dbdma: set FLUSH bit upon reception of flush command for unassigned DBDMA channels

This fixes MacOS 9 whereby it continually flushes and polls the status bits
until they are set to indicate a successful flush.

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>