cpuidle: dt: fix opencoded for_each_cpu() in idle_state_valid()

The function opencodes the for_each_cpu_from() by using an open for-loop.
Fix that in sake of readability.

While there, drop the 'valid' variable as it's pretty useless here.

Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Link: https://patch.msgid.link/20250604213908.27819-1-yury.norov@gmail.com


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