Bluetooth: BAP: Disallow bt_bap_stream_stop when CIS is not connected

When the CIS for a BAP stream is not connected, we cannot truly
be in the disabling state (the only state the stop operation
can be performed by the unicast client).

The reason for this is that if the CIS is disconnected, then
the ASCS server shall transition to the QoS Configured state
regardless of whether it has received a receiver stop ready
command from the unicast client.

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