Bluetooth: ISO: Fixes ISO central disconnect and cleanup issues

When an ISO channel is disconnect on the central, it is
not deallocated, but merely disconnected. This is because,
as per the HCI spec, the CIS handle lives on in the CIG.

Instead of unref'ing the bt_conn to 0, we simply put the
channel and connection in the disconnected state.

This also fixes a few missing returns for
terminating a CIG.

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