New Audio 2 Channel Microphone & Error Corrections for uac2_speaker_fb example

[pschatzmann]

I started to work on extending TinyUSB Auduino to provide the missing audio functionality.

Unfortunately I was struggeling, so I decided to go back to basics and confirm that the functionality is really working.

• I was missing a 2 channel microphone example. I think 2 channels are the most useful scenario, so I decided to add an example for this case. This will be the base for the Arduino functionality.
• Unfortunatly I could not make any of the speaker examples work. The uac2_headset example is not working and the uac2_speaker_fb was giving compile errors. So I have corrected the errors and made sure that it is compiling now properly.

The functionality has been tested on a RP2040.