Rename RGBW define to WS2812_RGBW

[fauxpark]

Description

This define really only affects the WS2812 driver:

• RGBLight only supports WS2812 and APA102 LEDs. As far as I can ascertain, there is no such thing as an RGBW APA102, and there is no support for RGBW in the current APA102 driver code.
• RGB Matrix supports WS2812 as well as a variety of ISSI LED drivers. These technically could do RGBW, as the drivers are simply mapping the RGB values to the PWM buffer indices specified in their LED config structs, but I would argue these should get their own defines (eg. IS31FL3733_RGBW etc.), if there exists a board with RGBW LEDs driven by one of these chips.

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).