Bluetooth: L2CAP_BR: Reject the conn req if sec levels do not match

If the ACL link has been encrypted and it has a authenticated link key,
it means the pairing procedure has been done. And the security level of
the link key can not be upgraded. In this case, if `conn->sec_level` is
less than the required security level of the L2CAP channel, reject the
L2CAP conn request.

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