memspeed: Write a fixed value

Otherwise we have at least an extra addition in the loop
which squews the result compared to the read loop.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

litex/soc/software/libbase/memtest.c

@@ -185,7 +185,7 @@ void memspeed(unsigned int *addr, unsigned long size, bool read_only)
 		timer0_update_value_write(1);
 		start = timer0_value_read();
 		for(i = 0; i < size/sz; i++) {
-			array[i] = i;
+			array[i] = -1ul;
 		}
 		timer0_update_value_write(1);
 		end = timer0_value_read();