fix: Drive usage_o low if not supported (#357)

**Problem**
This PR includes a solution for issue #356. The
`cc_stream_fifo_optimal_wrap` originally assigned an unknown state to
`usage_o` when `Depth == 32'd2`. This issue was first found when running
Spyglass checks into the `snitch_cluster` ([PR
#330](pulp-platform/snitch_cluster#330)).

**Solution**
This fix simply defines the signal state and drives it low if not
supported. The specific value of `usage_o` should not be problematic in
this case since the port is generally not connected when instantiated at
a higher hierachy.

Author: gbellocchi

src/cc_stream_fifo_optimal_wrap.sv

@@ -79,7 +79,7 @@ module cc_stream_fifo_optimal_wrap #(
         );
 
         // usage is not supported
-        assign usage_o = 'x;
+        assign usage_o = '0;
     end