Simple System: Correctly tie-off unused signals

Two unused bits of device_err were only assigned in an initial block,
and it not in Verilator. They should have been tied off always to avoid
mixed blocking/nonblocking assignments to the same signal.

examples/simple_system/rtl/ibex_simple_system.sv

@@ -78,7 +78,6 @@ module ibex_simple_system (
   `else
     initial begin
       rst_sys_n = 1'b0;
-      device_err = '{default:1'b0};
       #8
       rst_sys_n = 1'b1;
     end
@@ -88,6 +87,10 @@ module ibex_simple_system (
     end
   `endif
 
+  // Tie-off unused error signals
+  assign device_err[Ram] = 1'b0;
+  assign device_err[SimCtrl] = 1'b0;
+
   bus #(
     .NrDevices   (NrDevices),
     .NrHosts     (NrHosts  ),