Bluetooth: Use the stored LTK for restabilishing security

Now that it's possible that the exchanged key is present in
the link key list, we may be able to estabilish security with
an already existing key, without need to perform any SMP
procedure.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>