ao_wasapi: Support S/PDIF passthrough

Entirely untested as this troper has no S/PDIF hardware.

Refuses trying any other format if we can't use passthrough, or we would
end up sending white noise at the user.
ao_wasapi: Also do passthrough for AF_FORMAT_MPEG2

That's the sample format ad_spdif uses when the source is MP3.
ao_wasapi: Log the passthrough format in MSGL_V