Bluetooth: Update sec_level/auth_type for already existing connections

Update auth level for already existing connections if it is lower
than required by new connection.

Signed-off-by: Ville Tervo <ville.tervo@nokia.com>
Reviewed-by: Emeltchenko Andrei <andrei.emeltchenko@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@nokia.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>