OGR-OCI: use VARCHAR2 instead of VARCHAR for unsupported types
         see http://lists.osgeo.org/pipermail/gdal-dev/2014-April/038840.html


git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27274 f0d54148-0727-0410-94bb-9a71ac55c965