rtsold: lower log level of spurious message

[franco_opnsense.org]

Summary

When rtsold with `-d' it is supposed to show informal messages,
though it actually picks up prefix delegation RAs on other non-
configured interfaces. Since it's listening on a raw socket,
we get all those things, but there's no misconfiguration and
no interest in what happens with other interfaces per se and
the debug level shall be sufficient.

PR: https://forum.opnsense.org/index.php?topic=3392

[markj]

Makes sense to me.