usb hub: use flush_work instead of flush_work_sync

flush_work_sync and flush_work are now the same and flush_work_sync
has been deprecated. This fixes the following warning:

drivers/usb/core/hub.c: In function hub_quiesce:
drivers/usb/core/hub.c:1216:3: warning: flush_work_sync is deprecated (declared at include/linux/workqueue.h:448) [-Wdeprecated-declarations]
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NOctavian Purdila <octavian.purdila@intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/hub.c

@@ -1213,7 +1213,7 @@ static void hub_quiesce(struct usb_hub *hub, enum hub_quiescing_type type)
 	if (hub->has_indicators)
 		cancel_delayed_work_sync(&hub->leds);
 	if (hub->tt.hub)
-		flush_work_sync(&hub->tt.clear_work);
+		flush_work(&hub->tt.clear_work);
 }
 
 /* caller has locked the hub device */