writeback: Remove a mux leg on data_in

Initialize to 0 forces the mux to have an extra leg fed with zeros.

Instead initialize data_in to one of the mux inputs

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

writeback.vhdl

@@ -95,7 +95,7 @@ begin
         partial_write <= '0';
         sign_extend <= '0';
         second_word <= '0';
-        data_in <= (others => '0');
+        data_in <= e_in.write_data;
 
         if e_in.write_enable = '1' then
             w_out.write_reg <= e_in.write_reg;