cpuidle: teo: Simplify handling of total events count

Instead of computing the total events count from scratch every time,
decay it and add a PULSE value to it in teo_update().

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Tested-by: Christian Loehle <christian.loehle@arm.com>
Link: https://patch.msgid.link/9388883.CDJkKcVGEf@rjwysocki.net