NetworkPkg/Mtftp6Dxe: Correct the total received and saved block number.

NetworkPkg/Mtftp6Dxe: Correct the total received and saved block number.

The block returned from Mtftp6RemoveBlockNum is not the total received and
saved block number if it works in passive (Slave) mode.

The issue was exposed by the EMS test.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>