checkPortalSize function logic fix for setLimitBitrateByPortal

Created by: richardbushell

Bug as described in my Pull Request: https://github.com/Dash-Industry-Forum/dash.js/pull/2935 (Feedback Agreement has been emailed to Dash-IF Google Groups Email List)

When using setLimitBitrateByPortal, then currently the checkPortalSize function calculates and returns the wrong index due to a bug in its current logic, resulting in a lower quality representation being requested than appropriate. The PR cited above fixes that bug.