Bluetooth: Classic: SSP: Correct pairing method

In current implementation, if the MITM flag of both sides is disabled,
the pairing method is incorrect.

Such as, the IOCAP of both sides is `display_yesorno`, the pairing
method is `PASSKEY_CONFIRM`. But actually, it should be `JUST_WORKS`
in this case.

Fix the issue by setting the pairing method to `JUST_WORKS` if the
MITM flag of both sides is false.

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