writeback: Eliminate inferred latch

This initializes data_in to all zeroes so that it doesn't become a
set of 64 inferred latches.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

writeback.vhdl

@@ -95,6 +95,7 @@ begin
         partial_write <= '0';
         sign_extend <= '0';
         second_word <= '0';
+        data_in <= (others => '0');
 
         if e_in.write_enable = '1' then
             w_out.write_reg <= e_in.write_reg;