iio: Fix incorrect error debug in iioutils_get_param_float()

The "ret" variable was not set, so we'd end up with "failed: success".

src/iio-buffer-utils.c

@@ -211,6 +211,7 @@ iioutils_get_param_float (float      *output,
 		return 0;
 	}
 
+	ret = -errno;
 	g_debug ("Failed to read float from %s: %s", filename, g_strerror (-ret));
 	g_free (filename);