Bluetooth: Fix L2CAP socket shutdown for LE connections

During the L2CAP socket shutdown, the LE connection is not terminated
as expected. This patch fixes hci_conn_disconnect function so it can
be used to disconnect LE connections.

This bug can be reproduced using l2test tool. Just after the LE
connection is established, kill l2test and the LE connection
is not terminated.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Bluetooth: Rename hci_acl_disconn

As this function is able to disconnect both ACL and LE links,
renaming it to hci_disconnect is more suitable.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>