NetworkPkg: Fix bug related DAD issue in IP6 driver.

If we set PXEv6 as the first boot option and reboot immediately
after the first successful boot, it will assert. the root cause is
when we set the policy from manual to automatic in PXE driver,
the ip6 Configure item size is already set to zero and other
structures are also released, So it is not needed to perform DAD call
back function which is invoked by Ip6ConfigSetMaunualAddress.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>