Fix ps.test in --ps-protocol

mysql-test/t/ps.test:
  Disable prepared statements for a part of the test to avoid
  result mismatch.

mysql-test/t/ps.test

@@ -693,11 +693,13 @@ prepare stmt from "select ?FROM t1";
 prepare stmt from "select FROM t1 WHERE?=1";
 --error 1064
 prepare stmt from "update t1 set a=a+?WHERE 1";
+--disable_ps_protocol
 --error 1064
 select ?;
 --error 1064
 select ??;
 --error 1064
 select ? from t1;
+--enable_ps_protocol
 drop table t1;
 #