PM / Runtime: Correct documented return values for generic PM callbacks

As of commit 05aa55dd ("PM / Runtime:
Lenient generic runtime pm callbacks"), the generic power management
callbacks pm_generic_runtime_suspend() and pm_generic_runtime_resume()
return 0, not -EINVAL, if the device doesn't provide its own callbacks.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/power/runtime_pm.txt

@@ -667,11 +667,11 @@ driver/base/power/generic_ops.c:
 
   int pm_generic_runtime_suspend(struct device *dev);
     - invoke the ->runtime_suspend() callback provided by the driver of this
-      device and return its result, or return -EINVAL if not defined
+      device and return its result, or return 0 if not defined
 
   int pm_generic_runtime_resume(struct device *dev);
     - invoke the ->runtime_resume() callback provided by the driver of this
-      device and return its result, or return -EINVAL if not defined
+      device and return its result, or return 0 if not defined
 
   int pm_generic_suspend(struct device *dev);
     - if the device has not been suspended at run time, invoke the ->suspend()