Bluetooth: Add proper shutdown support to SCO sockets

The SCO sockets for Bluetooth audio setup and streaming are missing the
shutdown implementation. This hasn't been a problem so far, but with a
more deeper integration with PulseAudio it is important to shutdown SCO
sockets properly.

Also the Headset profile 1.2 has more detailed qualification tests that
require that SCO and RFCOMM channels are terminated in the right order. A
proper shutdown function is necessary for this.

Based on a report by Johan Hedberg <johan.hedberg@nokia.com>

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Tested-by: Johan Hedberg <johan.hedberg@nokia.com>