Bluetooth: Fix missing command complete for mgmt_load_long_term_keys

All management events are expected to indicate successful completion
through a command complete event, however  the load long term keys
command was missing this. This patch adds the missing event.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>