return wrong warning when to set tidb_gogc_tuner_max_value/tidb_gogc_tuner_min_value

[hawkingrei]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

 set global tidb_gogc_tuner_max_value=500;
 show warning;

2. What did you expect to see? (Required)

None

3. What did you see instead (Required)

| Level | Code | Message |
+---------+------+------------------------------------------------------------+
| Warning | 1292 | Truncated incorrect tidb_gogc_tuner_max_value value: '500' |
+---------+------+------------------------------------------------------------+

4. What is your TiDB version? (Required)