[RFC] Cleanup/refactor the usbaudio driver

[Daniel Mack <daniel@caiaq.de>]

I've spent some time cleaning up the usbaudio driver. I felt that the
existing implementation was quite difficult to follow because it
contained code for all sorts of things mixed up in one big file.

So I split the driver into pieces and tried to be as less intrusive as
I can, meaning that I didn't touch much real code but rather copied
the functions as they were and factored them out. Of course, non-static
functions were properly prefixed to avoid name space pollution.

I also moved almost all quirks to a seperate file and introduced two
more generic hooks to handle them, so the excecption add less to the
actual code of the driver.

Unfortunately it wasn't easily possible to put that big change into
smaller pieces, and so 3/3 became a huge chunk.

After all, I feel that it is easier to read now, but I don't know
whether you agree. So take this as RFC for now :)

Daniel