USB: serial: enable async suspend for usb serial port device

Usb serial port device is child of its usb interface device, so
we can enable async suspend of usb serial port device to speedup
system suspend.
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/usb-serial.c

@@ -1071,6 +1071,8 @@ int usb_serial_probe(struct usb_interface *interface,
 		dev_set_name(&port->dev, "ttyUSB%d", port->number);
 		dbg ("%s - registering %s", __func__, dev_name(&port->dev));
 		port->dev_state = PORT_REGISTERING;
+		device_enable_async_suspend(&port->dev);
+
 		retval = device_add(&port->dev);
 		if (retval) {
 			dev_err(&port->dev, "Error registering port device, "