UefiCpuPkg/MpInitLib: Rollback old change 2a5997f8.

UefiCpuPkg/MpInitLib: Rollback old change 2a5997f8.

In some special cases, after BSP sends Init-sipi-sipi signal
AP needs more time to start the Ap procedure. In this case
BSP may think AP has finished its task but in fact AP hasn't began
yet.

Rollback former change to keep the status which only be used
when AP really finished task.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>