Bluetooth: L2CAP: Only call disconnected callback for connected channels

If channel has conn set to NULL disconnected shall not be called as
may cause invalid actions as with bt_gatt_disconnected being called
with a NULL conn.

Change-Id: I11bc41a34c2a2a3bc5f8514ec4a948235473cfba
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>