Bluetooth: Return added key when adding LTKs and IRKs

The SMP code will need to postpone the mgmt event emission for the IRK
and LTKs. To avoid extra lookups at the end of the key distribution
simply return the added value from the add_ltk and add_irk functions.

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