platform/chrome: wilco_ec: Fix keyboard backlight probing

The EC on the Wilco platform responds with 0xFF to commands related to
the keyboard backlight on the absence of a keyboard backlight module.
This change allows the EC driver to continue loading even if the
backlight module is not present.

Fixes: 119a3cb6 ("platform/chrome: wilco_ec: Add keyboard backlight LED support")
Signed-off-by: Daniel Campello <campello@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Benson Leung <bleung@chromium.org>