Bluetooth: Audio: Fix issue with invalid bt_conn_ref in ASCS

In ascs.c we had a case where we assigned stream->conn
without taking the ref.

In bt_audio_stream_attach we did not check if stream->conn
was NULL before taking a reference, causing multiple calls
to ase_config to take multiple references.

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