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

MdeModulePkg/Include/Guid/SmmVariableCommon.h

@@ -1,7 +1,7 @@
 /** @file
   The file defined some common structures used for communicating between SMM variable module and SMM variable wrapper module.
 
-Copyright (c) 2011, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2011 - 2013, Intel Corporation. All rights reserved.<BR>
 This program and the accompanying materials are licensed and made available under 
 the terms and conditions of the BSD License that accompanies this distribution.  
 The full text of the license may be found at
@@ -87,7 +87,7 @@ typedef struct {
 ///
 typedef struct {
   EFI_GUID    Guid;
-  UINTN       NameSize;
+  UINTN       NameSize;     // Return name buffer size
   CHAR16      Name[1];
 } SMM_VARIABLE_COMMUNICATE_GET_NEXT_VARIABLE_NAME;