Bluetooth: Audio: Move the decode_base functionality to a public function

Moved the implementation of the pa_decode_base to a public
function, and modified it to return better error codes.

The implementation was moved to be publically
accessible, rather than internal in the Broadcast Sink,
as e.g. a Broadcast Assistant need to be able to decode the
BASE too. This wil also allow an application to decode it
itself, if it so wanted to.

This also adds a log module for the audio.c file, as logging
would otherwise not be supported.

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