Show correct current download and upload speed

Summary:
After recent change to use NM::DeviceStatistics instead of plasma dataengine, this part
of code wasn't changed and instead of showing current download/upload speed, it was
showing total downloaded/uploaded size

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16269