platform/chrome: cros_ec_lightbar - fix duplicate const warning

Fix the following sparse warning:

    drivers/platform/chrome/cros_ec_lightbar.c:254:25: sparse: duplicate const

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Olof Johansson <olofj@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Olof Johansson <olof@lixom.net>