trivial: fu-plugin: quiet down debug output for optionrom plugin
trivial: fu-plugin: quiet down debug output for optionrom plugin

All of these messages continually output are just noise for unsupported
devices:

```
18:11:52:0851 FuPlugin             using generic udev_device_added() on optionrom
18:11:52:0851 FuEngine             optionrom ignoring: no FWUPD_GUID property
```

hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd