platform/chrome: cros_ec_sysfs: introduce to_cros_ec_dev define.

Add a define to get the cros_ec_dev from device and use it.

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>