main/openrc: silence find warnings in hwdrivers service

During bootup /sys might not be fully populated. This can cause find to print warnings such as find: /sys/module/amd64_eadc no such file or directory which I see frequently during bootup. While the existing shell command silences modprobe warnings it does not silence find warnings. This commit adjusts the command accordingly to also redirect standard error to /dev/null for the find invocation.