fpga: lib: Fix pass-through mode in axi_fifo

A missing "else" after 6ea4e48 was causing two configurations to occur
at the same time for some values of SIZE.

This commit also sets the "space" and "occupied" outputs to 0 when in
pass-through mode.


Original-commit: 19b5c85b514e1c37238308e405108c69ecbddb7e

lib/fifo/axi_fifo.v

@@ -56,8 +56,10 @@ module axi_fifo #(
       assign o_tdata  = i_tdata;
       assign o_tvalid = i_tvalid;
       assign i_tready = o_tready;
+      assign space    = 16'h0;
+      assign occupied = 16'h0;
      end
-     if (SIZE == 0) begin : gen_fifo_flop
+     else if (SIZE == 0) begin : gen_fifo_flop
         axi_fifo_flop #(
           .WIDTH(WIDTH)
         ) fifo_flop (