IntelSiliconPkg/ShadowMicrocodePei: Fix return value in ShadowMicrocode.

ShadowMicrocode should return EFI_NOT_FOUND when no valid Microcode found
in FIT table.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3110



Signed-off-by: Aaron Li <aaron.li@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Cc: Rangasai V Chaganty <rangasai.v.chaganty@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>