Fix a bug for vlan ping failure.

Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com >
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jin Eric <eric.jin@intel.com>


git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15017 6f19259b-4bc3-4df7-8a09-765794883524