embassy-rp: Make bit-depth of I2S PIO program configurable

[embediver]

The PioI2sOutProgram had a fixed 16bit sample depth, but that isn't reflected in the types name or the documentation.

The PioI2sOut output driver on the other hand accepts a bit_depth argument.
This was previously only used to calculate the clock frequency, while no other values than 16 made any sense.

This MR implements setting the sample bit-depth by setting it to scratch register Y, from where it is read by the PIO program.

Also the channel argument is removed, since only 2 channels are supported.

[Dirbaio]

Also the channel argument is removed, since only 2 channels are supported.

what do you mean with this? Was "other than 2 channels" already unsupported before, or does it become unsupported with your changes?

If the latter, maybe we should find a way to make channel count and bit depth configurable.

[embediver]

Channel count wasn't really supported before and I don't think I2S could support it.
Atleast I don't know how I2S with only one channel would have to look, since a word clock (which is also the left/right clock) is mandatory.
And more than two channels is definitely not supported by standard I2S.

The channel argument was only used to calculate the clock frequency by calculating sample_rate * bit_depth * channel_count.
So now its just a fixed x2 in the calculation.