Bluetooth: Host: Unref ISO fragments after disconnection

When sending data using ISO and the data is fragmented, if the
connection is cut before all the fragments are sent, the data buffer
will be leaked.

Fix the issue by unref'ing the buffer when ISO is not in a connected
state.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>
(cherry picked from commit 3a098c9f)