usbnet: qmi_wwan: 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 both
usbnet_suspend() and subdriver->suspend() MUST return 0 in
system sleep context.
Acked-by: NDavid S. Miller <davem@davemloft.net>
Acked-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/usb/qmi_wwan.c

@@ -271,6 +271,11 @@ static int qmi_wwan_suspend(struct usb_interface *intf, pm_message_t message)
 	struct qmi_wwan_state *info = (void *)&dev->data;
 	int ret;
 
+	/*
+	 * Both usbnet_suspend() and subdriver->suspend() MUST return 0
+	 * in system sleep context, otherwise, the resume callback has
+	 * to recover device from previous suspend failure.
+	 */
 	ret = usbnet_suspend(intf, message);
 	if (ret < 0)
 		goto err;