Fix l/100 km to MPG conversion

Summary:
Previous code (probably) worked only for l/100 km -> MPG but not the other way around. Fix this by using reciprocal conversion when necessary.

Output:
38.7 MPG -> 6.077 l/100 km; 16.455 km/l; 46.482 MPGI
16.455 km/l -> 6.077 l/100 km; 38.7 MPG
46.48 MPGI -> 6.077 l/100 km

Seems good to me.

BUG: 378967

Reviewers: broulik, Frameworks, aacid

Subscribers: apol, aacid, meven, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20284