print wrong flen with the FLOAT

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

run a SQL and print column's FieldType like `col.Tp.GetFlen()`
```sql
CREATE TABLE `DBZ3865` (
`id` INT NOT NULL AUTO_INCREMENT,
`f4_23` FLOAT(23) DEFAULT NULL
)
```
### 2. What did you expect to see? (Required)
flen is 23
### 3. What did you see instead (Required)
flen is -1
### 4. What is your TiDB version? (Required)
master