Bluetooth: Set the correct security level for SC LTKs

When the looked-up LTK is one generated by Secure Connections pairing
the security level it gives is BT_SECURITY_FIPS. This patch updates the
LTK request event handler to correctly set this level.

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