fix playing mono files on stereo output

[dhalbert]

While testing #5932, I discovered a different problem, which is that mono files played back on SAMD51 on stereo channels caused a buzz every other time. This was due to not checking the number of channels when setting up the DMA's.

Now, a mono file will play in only one channel if there are stereo channels. If two DMA's are set up (depends on the channel order), then the mono file would play twice as fast (I guess double incrementing is going on). This may be fixable but I think requires some internal API changes.

This is not the perfect fix, but is an improvement over the current situation, because it gets rid of poor sound quality (buzz) when playing mono files on stereo.

[tannewt]

Thanks for fixing this! Looks like mono in -> stereo will require refactoring since the dma setup code also manages the input buffer that would need to be shared.