PM / runtime: Check ignore_children in pm_runtime_need_not_resume()

Modify pm_runtime_need_not_resume() to make it avoid taking
power.child_count for devices with power.ignore_children which
is consistent with the runtime PM usage of these fields.

Suggested-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>