UefiCpuPkg: Rollback field name changes

Roll back commit 56649f4301.
The original names follows the spec definition.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

UefiCpuPkg/Library/MpInitLib/MpLib.c

@@ -243,7 +243,7 @@ RestoreVolatileRegisters (
       VolatileRegisters->Tr < VolatileRegisters->Gdtr.Limit) {
     Tss = (IA32_TSS_DESCRIPTOR *)(VolatileRegisters->Gdtr.Base +
                                   VolatileRegisters->Tr);
-    if (Tss->Bits.Present == 1) {
+    if (Tss->Bits.P == 1) {
       Tss->Bits.Type &= 0xD;  // 1101 - Clear busy bit just in case
       AsmWriteTr (VolatileRegisters->Tr);
     }