Bluetooth: Classic: SMP: Set secure connection for derived LTK

If the encrypt value of classic connection is
`BT_HCI_ENCRYPTION_ON_BR_AES_CCM`, set the flag `BT_KEYS_SC` for
the derived LTK.

Or, clear the flag `BT_KEYS_SC` for the derived LTK.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>