PDMIn: turn serializer on when starting DMA #927
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Thought about this some more, and realized that synchronizing the turning on of the serializer with turning on DMA might solve the problem of lost triggers. The PDMIn clock to the microphone still runs all the time, to avoid microphone startup time (verified with Saleae), but the serializer is turned on and off as needed.
Tested on Metro M0 and M4, and works. I left in the hang prevention added in #916, because it's cheap, but tested with that temporarily turned off to make sure this new fix really does something. Rewrote #916 slightly to make it more compact.
So I think this really fixes #879.