USB: serial: comments on suspend failure

If suspend callback fails in system sleep context, usb core will
ignore the failure and let system sleep go ahead further, so
this patch comments on the case and requires that serial->type->suspend()
MUST return 0 in system sleep context.
Acked-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/usb-serial.c

@@ -1137,6 +1137,11 @@ int usb_serial_suspend(struct usb_interface *intf, pm_message_t message)
 
 	serial->suspending = 1;
 
+	/*
+	 * serial->type->suspend() MUST return 0 in system sleep context,
+	 * otherwise, the resume callback has to recover device from
+	 * previous suspend failure.
+	 */
 	if (serial->type->suspend) {
 		r = serial->type->suspend(serial, message);
 		if (r < 0) {