platform/chrome: cros_ec_lightbar - Avoid I2C xfer to EC during suspend

A Mutex lock in cros_ec_cmd_xfer which may be held by frozen
Userspace thread during system suspending. So should not
call this routine in suspend thread.

Signed-off-by: Jeffery Yu <jefferyy@nvidia.com>
Signed-off-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Benson Leung <bleung@chromium.org>