RP2040 PWMAudioOut: Release DMA channels after play has finished

[dhalbert]

Fix RP2040 PWMAudioOut stopping playing completely. It will stop playing after playing six samples.

"Scratchiness" and other audio quality issues are not necessarily addressed by this.

The essential problem was that the 12 DMA channels were being used up because the two that were being used were not freed after use, due to a missing

        audio_dma_stop(&self->dma);

in PWMAudioOut.c.

In the course of this I made some other more minor changes:

• Rename single_channel to single_channel_output to clarify its meaning. This changed a lot of files.
• Do pin claiming in PWMOut, not in PWMAudioOut.
• Fix up uses of DMA_CH0_CTRL_TRIG_EN_BITS and DMA_CH1_CTRL_TRIG_EN_BITS. They are actually the same value, but it's possible in some future processor they might not be, so I regularized the use.
• Reorder one struct's fields to save a bit of space.

[tannewt]

Good find! Thank you!

[dhalbert]

Fixed one failing build. Ready for re-review. I will look at crackling audio separately, since this fixes an immediate problem.

[dhalbert]

@kevinjwalters This may partially fix #4208, but I have not tested it for noisy output yet. I was testing with an artificial audio file with non-audible data to make sure the buffers were filling properly.

[jepler]

Makes sense, but didn't test.