platform/chrome: cros_ec_debugfs: Use octal permissions '0444'

Fixed the following checkpatch warning:

    WARNING: Symbolic permissions 'S_IRUGO' are not preferred. Consider
    using octal permissions '0444'.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Benson Leung <bleung@chromium.org>