Fix right side ws2812 leds having two indices

[daskygit]

Description

I'm assuming this is a bug but I'm unsure if targeting master is the correct option as it may break users keymaps depending on which index they used to address the right side leds.

So the example below shows the issue, where index 12 and 30 currently refers to the same physical led on the right side.

// config.h
RGB_MATRIX_SPLIT { 18, 18 }

// keymap.c
void rgb_matrix_indicators_user(void) {
    rgb_matrix_set_color(30, 0, 0, 255); // blue
    rgb_matrix_set_color(12, 255, 255, 255); // white
}

This currently results in the led being white but after applying this PR it's blue as you'd expect.

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).