Bluetooth: Fix exposing full value of shortened LTKs

When we notify user space of a new LTK or distribute an LTK to the
remote peer the value passed should be the shortened version so that
it's easy to compare values in various traces. The core spec also sets
the requirements for the shortening/masking as:

"The masking shall be done after generation and before being
distributed, used or stored."

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>