platform/chrome: cros_ec: Fix possible leak in led_rgb_store()

Function led_rgb_store() contains some direct returns in error cases that
leak the already allocated cros_ec_command message structure. Make sure
that 'msg' is freed in all exit paths. Detected by Coverity CID 1309666.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Olof Johansson <olof@lixom.net>