Update of readerfrontlight.lua.

The last code changes resulted in crashes. Now they don't and it the fine tuning is improved.

It changes `steps` in line 13 for better finetuning,
`rate` in line 22 so that it has a nil value for `Kobo_phoenix' 
and delta_int in line 48 to the last know working syntax.

Could someone check whether line 22 does result in 3.0 for non-phoenix devices?