Fix a bug that prevents Fat driver being unloaded successfully.

Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2-fatdriver2/code/trunk/FatPkg@82 efd2e655-3735-4b16-a529-16bfa2dd702b