Whitespace fixes in PixelDigitization code

This MR should contain only whitespace changes. I ran the uncrustify command:

/afs/cern.ch/atlas/offline/external/uncrustify/bin/uncrustify -c /afs/cern.ch/atlas/offline/external/uncrustify/atlas.cfg --no-backup *

on the folder.

The changes are needed since there is a mix of tabs and spaces used for the indentation making it very difficult to read.