Bluetooth: Update hci_add_link_key() to return pointer to key

By returning the added (or updated) key we pave the way for further
refactoring (in subsequent patches) that allows moving the mgmt event
sending out from this function (and thereby removal of the awkward
new_key parameter).

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