MdeModulePkg/Core/PiSmmCore/PiSmmCore.c: Fix compiler warning about variable set but unused

Fix compiler warning about variable set but unused by returning status
of last operation.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Garrett Kirkendall <garrett.kirkendall@amd.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16381 6f19259b-4bc3-4df7-8a09-765794883524