Fix underflow when current wpm > latest wpm

[jesus-ramos]

Description

Fixes an underflow caused by some unsigned math in the WPM calculation.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[drashna]

Just a heads up, #13902 may be a better long term solution, and the breaking changes cycle is coming up soon.

[jesus-ramos]

I must have missed that change when I searched open pulls somehow. That is probably the more correct solution here. Depends on when you want to merge those changes in, feel free to discard this one if you plan to merge that in soon.