Revert warning fix and promote chunk to unsigned

fb3c2d12 caused a regression where Hercules would output:
HHC00600E SCE file X: error in function read(): Bad address

Promote chunk to size_t (unsigned) which will get rid of the
warnings and don't truncate the values
Merge pull request #30 from bluecmd/scedasd-regression

Revert warning fix and promote chunk to unsigned