IntelSiliconPkg/IntelVTdDxe: Fix CLANG detected incorrect return

CreateSecondLevelPagingEntryTable() has a return type of
VTD_SECOND_LEVEL_PAGING_ENTRY * and an error condition
returns a value of NULL.

Change return value of EFI_SUCCESS (value 0) to NULL to
address CLANG compiler detection of incorrect return type.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Rangasai V Chaganty <rangasai.v.chaganty@intel.com>
Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Ashraf Ali S <ashraf.ali.s@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Sai Chaganty <rangasai.v.chaganty@intel.com>