PM: Add a switch for disabling/enabling asynchronous suspend/resume

Add sysfs attribute /sys/power/pm_async allowing the user space to
disable/enable asynchronous suspend/resume of devices.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
index 3b44c20..7e79201 100644
--- a/drivers/base/power/main.c
+++ b/drivers/base/power/main.c
@@ -201,7 +201,7 @@
 	if (!dev)
 		return;
 
-	if (async || dev->power.async_suspend)
+	if (async || (pm_async_enabled && dev->power.async_suspend))
 		wait_for_completion(&dev->power.completion);
 }
 
@@ -563,7 +563,8 @@
 {
 	INIT_COMPLETION(dev->power.completion);
 
-	if (dev->power.async_suspend && !pm_trace_is_enabled()) {
+	if (pm_async_enabled && dev->power.async_suspend
+	    && !pm_trace_is_enabled()) {
 		get_device(dev);
 		async_schedule(async_resume, dev);
 		return 0;
@@ -867,7 +868,7 @@
 {
 	INIT_COMPLETION(dev->power.completion);
 
-	if (dev->power.async_suspend) {
+	if (pm_async_enabled && dev->power.async_suspend) {
 		get_device(dev);
 		async_schedule(async_suspend, dev);
 		return 0;