trivial: Downgrade a warning to a debug
trivial: Downgrade a warning to a debug

This cleans up the output when running fwupdtool with --plugin-whitelist...

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