Bluetooth: Use the link key list to temporarily store the STK

With this we can use only one place to store all keys, without
need to use a field in the connection structure for this
purpose.

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