Bugfix for pilot blade enabling/disabling in digitizer

This is an urgent patch for PR #3754.
Explanation:
In the /DataFormats/TrackerCommon/TrackerTopologys pxfDisk () method  the startbit for layer is the same as for the disk, therefore when the code checks the disk == 3, the statement is also true for layer ==3, which need to be fixed as soon as possible.