atdec: Set a channel mask for channel counts greater than 2.

Encountered when trying to play back a 6-channel mp4a file on an Apple computer.

There are certainly plenty of ways in which I'm not sure that this is architecturally the right way to implement this. Copying gst_audio_channel_position_to_core_audio() for instances seems decidedly undesirable, but it's not clear to me that there's a nice way to share it.

Thanks Tim-Philipp Müller for helping to point out the proper place to fix the problem.