Fixes for is31fl3729 LED matrix driver off-by-one errors

[ms-eevee]

This PR fixes a couple latent bugs within the is31fl3729 matrix driver since #23109 and #21944, resulting in LEDs on sink line CS1 and the last sink line defined by the user to not light up.

Description

First, within is31fl3729_write_pwm_buffer, less or equal in the for loop causes the write to overrun the bounds of pwm_buffer, as well as clobbering the scaling register due to the address auto-increment on concurrent bytes. Changing to less-than results in keeping in bounds, from 0x00 to 0x8E.

Second, within is31fl3729_set_scaling_register, the calculation for the scaling register address hadn't changed after the changes to the pin definitions, causing scaling register writes for LEDs to be written starting at 0x8F (effectively setting SW9_CS16 LED to max bright by default) through to 0x9E , with writes stopping one short of the highest CS value register, as well as never being able to write to scaling register 0x9F.

These issues were noticed as I'm building my own custom split keeb using this matrix driver; If needed, I would be happy to provide schematics, PCB layout, and pictures as well 😊

Thank you, QMK team!

Types of Changes

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