[Minor] Cleanup siac worker output

MINOR MERGE REQUEST

MR Guidelines

Overview

Sanitize the upload cooldown time for siac renter workers so that is is only ever positive.

Example for Visual Changes

This Branch: https://asciinema.org/a/Pvet33L0khBsX62cpT5dn69EG

Master: https://asciinema.org/a/0ApFqMW8mycs37Zt24SrY3oKf