samples: Bluetooth: Few fixups for USB data for broadcast audio sink

This commit does several but minor changes to the USB handling of
the broadcast audio sink, such as improving logging, removing a few
unncessary pieces of code and some minor performance improvements.

It removes some uses of the ring buffers, which effectively clears
up around 30KB of RAM, while also reducing how much memory is being
copied, improving performance.

Some of this removes the existing code for partial support for
stereo, but that code did not work in the first place. Proper stereo
support will be added in a later commit.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>