UefiCpuPkg/MpInitLib: Always initialize the DoDecrement variable

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2901



The DoDecrement variable in ApWakeupFunction () wasn't always being
initialized. Update the code to always fully initialize it.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <76a9f18992475b915e5f8457704676067210cacf.1597935198.git.thomas.lendacky@amd.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Tested-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>