Add type cast on variable before operation.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Eric Dong <Eric.Dong@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/IntelFrameworkModulePkg@15778 6f19259b-4bc3-4df7-8a09-765794883524