bios/sdram: make read leveling robust for KUS SDRAM

Increases the initial delay step into the valid read window as
with the original delay I was not getting out of the noisy
transition window, as evidenced by seeing read delay windows
of only 8 LSB ~10% of the time, leading to failing memory
tests

litex/soc/software/bios/sdram.c

@@ -383,7 +383,7 @@ static void read_delays(void)
 
 		/* Get a bit further into the working zone */
 #ifdef KUSDDRPHY
-		for(j=0;j<8;j++) {
+		for(j=0;j<16;j++) {
 			delay += 1;
 			ddrphy_rdly_dq_inc_write(1);
 		}