Bluetooth: Use the LTK after receiving a LE Security Request

When receiving a security request from the remote device we should find
if there is already a LTK associated with the remote device, if found
we should use it to encrypt the link.

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