platform/chrome: cros_kbd_led_backlight: switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management. Also change the term 'Chrome OS' for 'ChromeOS' to be
coherent with other drivers.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>