Bluetooth: Fix old_key_type logic for non-persistent keys

Even if there's no previous key stored the connection might still be
secured with a non-persistent key and in that case the key type in the
hci_conn struct should be checked.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>