platform/chrome: Use to_cros_ec_dev more broadly

Move to_cros_ec_dev macro to cros_ec.h and use it when the private ec
object is needed from device object.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Benson Leung <bleung@chromium.org>