PM: sleep: Adjust check before setting power.must_resume

PM: sleep: Adjust check before setting power.must_resume

[ Upstream commit eeb87d17aceab7803a5a5bcb6cf2817b745157cf ]

The check before setting power.must_resume in device_suspend_noirq()
does not take power.child_count into account, but it should do that, so
use pm_runtime_need_not_resume() in it for this purpose and adjust the
comment next to it accordingly.

Fixes: 107d47b2b95e ("PM: sleep: core: Simplify the SMART_SUSPEND flag handling")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/3353728.44csPzL39Z@rjwysocki.net
Signed-off-by: Sasha Levin <sashal@kernel.org>