Fixed warnings in libmtdutils

A few signed/unsigned comparison warnings were all that stood in the way
of a clean clang build.

Change-Id: Id8c47fcfa500a6ac164bddb855831cdc17d99847
Author: Edwin Vane <edwin.vane@intel.com>
Reviewed-by: Tareq A Siraj <tareq.a.siraj@intel.com>