platform/chrome: cros_ec_sysfs: use permission-specific DEVICE_ATTR variants

Use DEVICE_ATTR variants for read/write attributes. This simplifies the
source code, improves readbility, and reduces the chance of
inconsistencies.

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