LED drivers: remove PWM register offsets

[fauxpark]

Description

Offset should only be applied when actually writing out the buffer. This simplifies assigning PWM values to indices into the buffer as there is no need to subtract the offset first (and three times for R,G,B).

Additionally, some usages of int have been changed to uint8_t since they will never be negative nor greater than 255.

Lastly the +1 offset was removed from the 3742A driver; its PWM registers start at 0x00 unlike the other ex-IS31COMMON drivers.

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).