Bluetooth: Allow re-encryption with LTK when STK is in use

If we're encrypted with the STK we should allow re-encryption with an
LTK even though the achieved security level is the same. This patch adds
the necessary logic to the smp_sufficient_security function which is
used to determine whether to proceed with encryption or not.

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