led: Return -ENODEV if the LED device cannot be found

We normally use -ENODEV for a missing device, rather than -ENOENT. The
latter is reserved for when we have a device but cannot find something
within it.

Also avoid looking at the root LED device since it is only a container.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/led.h

@@ -35,7 +35,7 @@ struct led_ops {
  *
  * @label:	LED label to look up
  * @devp:	Returns the associated device, if found
- * @return 0 if found, -ve on error
+ * @return 0 if found, -ENODEV if not found, other -ve on error
  */
 int led_get_by_label(const char *label, struct udevice **devp);