iio-buffer-utils: Don't throw warnings on ENOENT

As we don't return when the file doesn't exist, don't print a warning
either, as this would pollute logs and throw people off when debugging
real problems.

Closes: #180

src/iio-buffer-utils.c

@@ -177,6 +177,7 @@ iioutils_get_param_float (float      *output,
 		fclose (sysfsfp);
 	} else {
 		ret = -errno;
+		if (ret != -ENOENT)
 			g_warning ("Failed to read float from %s", filename);
 	}