platform/chrome: cros_ec_lpc - Use existing function to check EC result

Commit 6db07b63 ("mfd: cros_ec: Check result code from EC messages")
added a common cros_ec_check_result() function that can be used to check
the ec_msg->result for errors and warns about them.

Use the existing function instead of duplicating same check in the driver.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Tested-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Olof Johansson <olof@lixom.net>