UefiCpuPkg/DxeMpLib: Fix bug when getting target C-State from eax

UefiCpuPkg/DxeMpLib: Fix bug when getting target C-State from eax

AP will get target C-State from eax[7:4]. We do shift in ebx firstly before set
to eax. It will lead ebx is incorrect in the next time.

The fix is to set ebx to eax firstly and does shift in eax. Thus, ebx could keep
original value.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>