Avoid potential race condtion with QFutureWatcher in FWUPD backend

Summary: One should connect to the finish slot before starting the threaded job.

Test Plan:
Ran
Opened FW updates
Showed none as before, no other issues

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16812