bluetooth: Config MTU transport after acquire

bluetooth: Config MTU transport after acquire

The configuration of the transport that depends on the MTU should be
performed every time the transport has been acquired, since the
parameters depend on what the Media API provides. This requires to
update the parameters of the sinks and sources as well.

This patch moves this code into a new function that will be called
when the stream is starting (setup_stream), from the IO thread.

This makes the code more robust, since the existing multiple calls to
bt_transport_acquire() do not rely on setup_bt() being able to acquire
the transport.