input sw thru right side broke at some point

reproduce:
- plug sound source into line in
- plug stereo headphones into hp out
- run:

>>> audio.input_thru_set_mute(0)
>>> audio.input_thru_set_volume_dB(0)
>>> audio.input_set_source(audio.INPUT_SOURCE_LINE_IN)

you now should hear the audio source on the left hp side while the right side is silent. bl00mbox does output on both channels which narrows things down a bit.

we distinctly remember that this worked back in d34d93c5 when the feature was introduced bc we had to hunt down a bug for quite a while, so you can diff the refactor from there.

ps: we didn't check if hw thru works. best to check before trying to fix anything. if that doesn't work either might be audio codec config (or hw itself, but let's hope it's not :3)