DHCP6 bug fix:

DHCP6 won’t process more message if one message’s Xid is mismatched.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16832 6f19259b-4bc3-4df7-8a09-765794883524