usb: Return -ENOSYS when system call is not available

Update usb_gadget_release() to use -ENOSYS, which is the correct error
code for U-Boot.

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

drivers/usb/gadget/udc/udc-uclass.c

@@ -45,7 +45,7 @@ int usb_gadget_release(int index)
 		dev_array[index] = NULL;
 	return ret;
 #else
-	return -ENOTSUPP;
+	return -ENOSYS;
 #endif
 }