platform/chrome: Make i2c_adapter_names static

Not used outside of the file, so declaration should be static. Picked up by
sparse:

drivers/platform/chrome/chromeos_laptop.c:44:12: warning: symbol
    'i2c_adapter_names' was not declared. Should it be static?

Signed-off-by: Olof Johansson <olof@lixom.net>
Reviewed-by: Benson Leung <bleung@chromium.org>