Bluetooth: Classic: SSP: Only set MITM when sec level is more than 2

The local device shall only set the MITM protection required flag if
the local device itself requires MITM protection.

Only set MITM flag when the required security level is more than 2 and
pairing method is not `JUST_WORKS`.

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