MdeModulePkg/Mtftp4Dxe: Fix the incorrect return status.

MdeModulePkg/Mtftp4Dxe: Fix the incorrect return status.

The incorrect return status was caused by the commit of 39b0867d, which
was to resolve the token status error that does not compliance with spec
definition, but it results the protocol status not compliance with spec
definition.

This patch is to resolve above issue.

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