Bluetooth: audio: ascs: Make stream->ep valid in bt_bap_stream_ops.released

Once the application bt_bap_stream_ops.released callback is called make
sure the stream->ep pointer is valid so that application can still access
the endpoint details like e.g. endpoint direction.

Fixes: ASCS/SR/ACP/BV-{24,25,26,27,28,29,30,31}-C
Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>