fix rgb_matrix flicker on split keyboards caused by RGB_DISABLE_TIMEOUT

[eberthold]

Description

Workaround for a bug which causes the slave side of a split keyboard (crkbd in my case) to randomly turn the rgb matrix on and off after every keypress when RGB_DISABLE_TIMEOUT is set. (had values like 300000 and 450000 for 5 or 7.5 minutes timeout)

I think the root of this bug has is in sync_timer.c where

qmk_firmware/tmk_core/common/sync_timer.c

Line 36 in 6e8eb2c

sync_timer_ms = time - timer_read32();

can produce a negative value and the result in

qmk_firmware/tmk_core/common/sync_timer.c

Line 46 in 6e8eb2c

return sync_timer_ms + timer_read32();

can still be negative, which then results in a range underflow of the uint32_t result and therefore will be a very large number.

So as a result the code in rgb_matrix.c will eventually go into the if case in line 295 and 296 and make the slave side think the timeout is reached.

I assume the flicker occurs because the calculation oscillate around 0 and sometimes is negative and sometimes positive.

This is just a workaround and only works 99% of the time - some short random flickers still occur.
My C skills and microcontroller knowledge are very rusted/limited so I currently wasn't able to figure out a real solution to fix the root of this problem

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

There is no issue right now for this

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).