Bluetooth: Audio: Remove const for bt_audio_stream_reconfig codec

The codec is assigned to the stream. However since the metadata,
which is stored in the codec, may be modified by other means,
the stored pointer cannot be const, and thus the
codec argument to bt_audio_stream_reconfig cannot be const
either.

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