Bluetooth: audio: ascs: Fix missing ISO state check

This fixes missing ISO connection state check, as there might be no ISO
connection when Receiver Stop Ready has been received. This might happen
on ASE state transition from Enabling to Disabling.
Without this check the code asserts in ascs_disconnect_stream_work_handler.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>