code: Cleanup and warning fixes from the Vulkan PR (#6163)

Co-authored-by: emufan4568 <geoster3d@gmail.com>
Co-authored-by: Kyle Kienapfel <Docteh@users.noreply.github.com>

src/core/hle/service/fs/fs_user.cpp

@@ -884,6 +884,8 @@ ResultVal<u16> FS_USER::GetSpecialContentIndexFromTMD(MediaType media_type, u64
     default:
         ASSERT(false);
     }
+
+    return ResultCode(-1);
 }
 
 FS_USER::FS_USER(Core::System& system)