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>

diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
index 443468e..2a982e6 100644
--- a/drivers/usb/serial/usb-serial.c
+++ b/drivers/usb/serial/usb-serial.c
@@ -1071,6 +1071,8 @@
 		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, "