PM / Runtime: Fix comments to match runtime callback code

Commit 05aa55dddb9ee4045c320661068bea78dad6a6e5 changed routines to
succeed if the driver handler is not defined. Comments were not updated.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

drivers/base/power/generic_ops.c

@@ -39,7 +39,7 @@ EXPORT_SYMBOL_GPL(pm_generic_runtime_idle);
  *
  * If PM operations are defined for the @dev's driver and they include
  * ->runtime_suspend(), execute it and return its error code.  Otherwise,
- * return -EINVAL.
+ * return 0.
  */
 int pm_generic_runtime_suspend(struct device *dev)
 {
@@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(pm_generic_runtime_suspend);
  *
  * If PM operations are defined for the @dev's driver and they include
  * ->runtime_resume(), execute it and return its error code.  Otherwise,
- * return -EINVAL.
+ * return 0.
  */
 int pm_generic_runtime_resume(struct device *dev)
 {