PM: Prevent waiting forever on asynchronous resume after abort

Only wait on a parent device during resume if the parent device is
suspended.

Consider three drivers, A, B, and C.  The parent of A is C, and C
has async_suspend set.  On boot, C->power.completion is initialized
to 0.

During the first suspend:
suspend_devices_and_enter(...)
 dpm_resume(...)
  device_suspend(A)
  device_suspend(B) returns error, aborts suspend
 dpm_resume_end(...)
   dpm_resume(...)
    device_resume(A)
     dpm_wait(A->parent == C)
      wait_for_completion(C->power.completion)

The wait_for_completion will never complete, because
complete_all(C->power.completion) will only be called from
device_suspend(C) or device_resume(C), neither of which is called
if suspend is aborted before C.

After a successful suspend->resume cycle, where B doesn't abort
suspend, C->power.completion is left in the completed state by the
call to device_resume(C), and the same call path will work if B
aborts suspend.

Signed-off-by: Colin Cross <ccross@android.com>

drivers/base/power/main.c

@@ -528,7 +528,8 @@ static int device_resume(struct device *dev, pm_message_t state, bool async)
 	TRACE_DEVICE(dev);
 	TRACE_RESUME(0);
 
-	dpm_wait(dev->parent, async);
+	if (dev->parent && dev->parent->power.status >= DPM_OFF)
+		dpm_wait(dev->parent, async);
 	device_lock(dev);
 
 	dev->power.status = DPM_RESUMING;