Bluetooth: fix shutdown on SCO sockets

shutdown should wait for SCO link to be properly disconnected before
detroying the socket, otherwise an application using the socket may
assume link is properly disconnected before it really happens which
can be a problem when e.g synchronizing profile switch.

Signed-off-by: Luiz Augusto von Dentz <luiz.dentz-von@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>