platform/chrome: cros_ec_lightbar: Use cros_ec_cmd_xfer_status helper

platform/chrome: cros_ec_lightbar: Use cros_ec_cmd_xfer_status helper

This patch makes use of cros_ec_cmd_xfer_status() instead of
cros_ec_cmd_xfer(). It allows us to remove some redundand code. In this
case, though, we are changing a bit the behaviour because of returning
-EINVAL on protocol error we propagate the error return for
cros_ec_cmd_xfer_status() function, but I think it will be fine, even
more clear as we don't mask the Linux error code.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Prashant Malani <pmalani@chromium.org>