[3.0] audiounit: fix surround input on stereo output
setPreferredOutputNumberOfChannels can fail, don't configure more channels than accepted. The OS will drop the extra channels instead of downmixing it.
When I first tested it, the apple TV was configured to always accept surround even with a stereo output (Change Format = Off) but this is not necessarily the case.
(cherry picked from commit 6c08c24b) Signed-off-by: Thomas Guillem thomas@gllm.fr