Scale brigthness for VIA

[yiancar]

This fix is to scale the rgb_matrix brightness from and to VIA in case the max brightness set in firmware is not 255.

We have had confusion from users in an upcoming keyboard testing round about this.

VIA expects a value from 0 to 255. If the max brigthness in the firmware is set to below 255, any value send from VIA to the keyboard above the max firmware brightness will clip to the max firmware brightness. This results in the VIA slider to feel unresponsive.

As we are already doing something similar for Effect Speed I think this is appropriate.

EDIT:
This is also already done for backlight, so I cleaned it up a bit and did the same for rgblight

Description

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).

[tzarc]

Swapped to develop due to core changes.