Fix wrong Dimmer behavior introduced with #6799 when SetOption37 < 128

[s-hadinger]

Description:

Bug reported by @blakadder on Discord.

Now Dimmer has the following behavior:

• If SetOption37 < 128 (default behavior), RGB and White are linked, i.e. only one of both can be On at the same time. Dimmer has the same behavior as before, it will change the dimmer value for the current mode, either RGB or White.
• If SetOption37 >= 128 (also called unlinked), RGB and White/CWWW are treated as two independant channels. Dimmer or Dimmer1 will change the Dimmer for RGB. Dimmer2 will change the Dimmer for white(s). Dimmer0 will change both Dimmers at once.

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR.
• The code change is tested and works on core pre-2.6
• The code change pass travis tests. Your PR cannot be merged unless tests pass
• I accept the CLA.