Clear/reset unsuccessful attempts
Solution 1:
What you want to express is something like: "Set the displayed count of failed login attempts to zero". Briefly.
The best way to do that depends on the design of the UI. For example, what other information is displayed. Is this button grouped with other items that are related to the login attempts?
If so, following the model of my kitchen scales, it could just say "Zero". Or, if you have space and want to be more explicit: "Set to zero".