Fix returning supported color modes

[hyperb1iss]

• Return a set including ColorMode.ONOFF instead of a list

[make-all]

Thanks for the effort of submitting a fix.
I've installed a slightly simpler version of the change which only changes the lists to sets without altering the logic.
ONOFF is actually handled already for the devices that support on/off and nothing else. The fallback empty set should not actually be used by any real devices, as it represents a light with no controls.