NetworkPkg: Correct the proxy DHCP offer handing

NetworkPkg: Correct the proxy DHCP offer handing

When PXE10/WFM11a offer received, we should only cache
the first PXE10/WFM11a offer, and discard the others. But
Current we discard all PXE10/WFM11a offer. This patch is
to fix this issue.

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