cpuidle: teo: Exclude cpuidle overhead from computations

One purpose of the computations in teo_update() is to determine
whether or not the (saved) time till the next timer event and the
measured idle duration fall into the same "bin", so avoid using
values that include the cpuidle overhead to obtain the latter.

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