Improve $spans calculation in cPanel layout

The cPanel layout calculates the `spans` to start a new line if it goes over 12. This works fine as long as `bootstrapSize` is not 0. If one or more module is set to 0, the design goes crazy because it doesn't start a new line, but the module is shown with full width.

This PR sets the bootstrapSize to 12 for the calculation so a new line is properly created.

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32446