dm: usb: Deprecate usb_get_dev_index()

This function should not be used with driver model. While there are users
of USB Ethernet that use driver model for USB but not Ethernet, we have
to keep it around. Add a comment to that effect.
Signed-off-by: NSimon Glass <sjg@chromium.org>

drivers/usb/host/usb-uclass.c

@@ -279,6 +279,14 @@ int usb_init(void)
 	return usb_started ? 0 : -1;
 }
 
+/*
+ * TODO(sjg@chromium.org): Remove this legacy function. At present it is needed
+ * to support boards which use driver model for USB but not Ethernet, and want
+ * to use USB Ethernet.
+ *
+ * The #if clause is here to ensure that remains the only case.
+ */
+#if !defined(CONFIG_DM_ETH) && defined(CONFIG_USB_HOST_ETHER)
 static struct usb_device *find_child_devnum(struct udevice *parent, int devnum)
 {
 	struct usb_device *udev;
@@ -312,6 +320,7 @@ struct usb_device *usb_get_dev_index(struct udevice *bus, int index)
 
 	return find_child_devnum(dev, devnum);
 }
+#endif
 
 int usb_post_bind(struct udevice *dev)
 {