PM / runtime: Allow no callbacks in pm_runtime_force_suspend|resume()

The pm_runtime_force_suspend|resume() helpers currently requires the device
to at some level (PM domain, bus, etc), have the ->runtime_suspend|resume()
callbacks assigned for it, else -ENOSYS is returned as an error.

However, there are no reason for this requirement, so let's simply remove
it by allowing these callbacks to be NULL.

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

drivers/base/power/runtime.c

@@ -1640,7 +1640,7 @@ static bool pm_runtime_need_not_resume(struct device *dev)
 int pm_runtime_force_suspend(struct device *dev)
 {
 	int (*callback)(struct device *);
-	int ret = 0;
+	int ret;
 
 	pm_runtime_disable(dev);
 	if (pm_runtime_status_suspended(dev))
@@ -1648,12 +1648,7 @@ int pm_runtime_force_suspend(struct device *dev)
 
 	callback = RPM_GET_CALLBACK(dev, runtime_suspend);
 
-	if (!callback) {
-		ret = -ENOSYS;
-		goto err;
-	}
-
-	ret = callback(dev);
+	ret = callback ? callback(dev) : 0;
 	if (ret)
 		goto err;
 
@@ -1704,7 +1699,7 @@ int pm_runtime_force_resume(struct device *dev)
 
 	callback = RPM_GET_CALLBACK(dev, runtime_resume);
 
-	ret = callback ? callback(dev) : -ENOSYS;
+	ret = callback ? callback(dev) : 0;
 	if (ret) {
 		pm_runtime_set_suspended(dev);
 		goto out;