NetworkPkg/UefiPxeBcDxe: Correct the handle for PXE Base Code Callback Protocol.

NetworkPkg/UefiPxeBcDxe: Correct the handle for PXE Base Code Callback Protocol.

According UEFI Spec:
The PXE Base Code Callback Protocol must be on the same handle as the PXE
Base Code Protocol.

But current implementation doesn't follow that. This patch is fix that issue.

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