PR: https://github.com/truenas/middleware/pull/8234

This commit fixes an issue where when system boots with a serial port configured in the grub configuration, if we disable the serial port once the system has booted - for enable check it says `enabled-runtime` instead of saying `disabled`. Now this gives the impression that this systemd service will start when the system reboots but that does not hold as grub is updated when we make such changes and on subsequent reboot we do not pass serial console parameters which results in systemd unit for that serial port correctly saying `disabled`. So the gist is, that instead of relying on returncode, we explicitly check for enabled keyword when seeing if service is enabled or not after it has been disabled via middleware.