Update SMM variable DXE driver GetNextVariable interface to comply with UEFI spec

Update SMM variable DXE driver GetNextVariable interface to comply with UEFI spec
VariableNameSize is the returned buffer size. GetNextVariable should behavior correct if it is bigger than SMM communication buffer or less than string size of VariableName.

Signed-off-by: Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by  : Dong Guo   <guo.dong@intel.com>
Reviewed-by  : Fu Siyuan  <siyuan.fu@intel.com>
Reviewed-by  : Zeng Star  <star.zeng@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14258 6f19259b-4bc3-4df7-8a09-765794883524