Interpreter: fix undefined bits of mffs

Lioncash tested this on hardware.

Source/Core/Core/PowerPC/Interpreter/Interpreter_SystemRegisters.cpp

@@ -448,11 +448,10 @@ void Interpreter::mcrfs(UGeckoInstruction _inst)
 void Interpreter::mffsx(UGeckoInstruction _inst)
 {
 	// load from FPSCR
-	// This may or may not be accurate - but better than nothing, I guess
 	// TODO(ector): grab all overflow flags etc and set them in FPSCR
 
 	UpdateFPSCR();
-	riPS0(_inst.FD) = (u64)FPSCR.Hex;
+	riPS0(_inst.FD) = 0xFFF8000000000000 | FPSCR.Hex;
 
 	if (_inst.Rc)
 		PanicAlert("mffsx: inst_.Rc");