usb: Return -ENOENT when no devices are found

When USB finds no devices it currently returns -EPERM which bootstd does
not understand. This causes other bootdevs of the same priority to be
skipped.

Fix this by returning the correct error code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/usb.h b/include/usb.h
index 42b001c..09e3f0c 100644
--- a/include/usb.h
+++ b/include/usb.h
@@ -257,7 +257,14 @@
 
 #endif
 /* routines */
-int usb_init(void); /* initialize the USB Controller */
+
+/*
+ * usb_init() - initialize the USB Controllers
+ *
+ * Returns: 0 if OK, -ENOENT if there are no USB devices
+ */
+int usb_init(void);
+
 int usb_stop(void); /* stop the USB Controller */
 int usb_detect_change(void); /* detect if a USB device has been (un)plugged */