[Bluetooth] Disconnect L2CAP connection after last RFCOMM DLC

The RFCOMM specification says that the device closing the last DLC on
a particular session is responsible for closing the multiplexer by
closing the corresponding L2CAP channel.

Signed-off-by: Ville Tervo <ville.tervo@nokia.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>