Bluetooth: Fix SMP security elevation from medium to high

If we have an unauthenticated key it is not sufficient to acheive high
security. Therefore, when deciding whether to encrypt the link or
request pairing, it is essential to in addition to checking the
existence of a key to also check whether it is authenticated or not.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>