Bluetooth: Use the updated key structures for handling LTKs

This updates all the users of the older way, that was using the
link_keys list to store the SMP keys, to use the new way.

This includes defining new types for the keys, we have a type for each
combination of STK/LTK and Master/Slave.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>