Bluetooth: Fix trying LTK re-encryption when we don't have an LTK

In the case that the key distribution bits cause us not to generate a
local LTK we should not try to re-encrypt if we're currently encrypted
with an STK. This patch fixes the check for this in the
smp_sufficient_security function.

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