platform/chrome: cros_ec_lightbar - hide unused PM functions

platform/chrome: cros_ec_lightbar - hide unused PM functions

The only reference to the new functions is inside of an #ifdef,
which now causes a harmless warning when CONFIG_PM_SLEEP is not set:

chrome/cros_ec_dev.c:478:12: error: 'ec_device_resume' defined but not used [-Werror=unused-function]
chrome/cros_ec_dev.c:469:12: error: 'ec_device_suspend' defined but not used [-Werror=unused-function]

This marks the two functions as __maybe_unused so they can get
silently dropped by the compiler.

Fixes: 405c84308c43 ("platform/chrome: cros_ec_lightbar - Control of suspend/resume lightbar sequence")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Benson Leung <bleung@chromium.org>