[rvfi] Only catch LSU RF write data if it's actually valid

Previously, the interface was also catching the data returned by stores
which may be X.

Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>

rtl/ibex_core.sv

@@ -1643,7 +1643,7 @@ module ibex_core import ibex_pkg::*; #(
 
   // Capture read data from LSU when it becomes valid
   always_comb begin
-    if (lsu_resp_valid) begin
+    if (lsu_rdata_valid) begin
       rvfi_mem_rdata_d = rf_wdata_lsu;
     end else begin
       rvfi_mem_rdata_d = rvfi_mem_rdata_q;