"Use site name as OTP QR code name prefix." config not saving correctly

Problem/Motivation

Every time the TFA settings are saved, the "Use site name as OTP QR code name prefix." toggles from TRUE to FALSE and vice-versa.

Steps to reproduce

1. Observe the currrent state of the "Use site name as OTP QR code name prefix." checkbox.
2. Without changing anyting, save the form.
3. Notice that the checkbox has toggled to the other boolean value.
4. Save the form again.
5. Notice that the checkbox has toggled to the original value.

Proposed resolution

Prevent this behaviour.