tuna: power: make sure we don't apply the screen off freq on screen on

This fixes issues when pressing the power button repeatedly in a short
period of time. The "on" call might not run, and when we re-read the
value on screen off to store it, we will be actually reading the capped
screen off value. Make sure that's not the case.
Review power freq on screen off code

Review general code
removed non sense comparation  "on ?" since is extra

Change-Id: I285368dbb449e4234e9cea8846c087fc4f3200f3