MdeModulePkg: Fixed two issues when error occurs in Mtftp4Start.

MdeModulePkg: Fixed two issues when error occurs in Mtftp4Start.

* This function sets returned status as token status and signal token
  when error occurs, and it results token status not compliance with
  spec definition. This patch fixed this issue.
* This function restore Tpl twice when Mtftp4WrqStart() returns an
  error, this patch fixed this issue.

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