cpuidle: teo: Cosmetic modifications of teo_update()

Rename a local variable in teo_update() so that its purpose is better
reflected by its name and use one more local variable in the loop
over the CPU idle states in that function to make the code somewhat
easier to read.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>