[Bug Report] Cascader Component restrict multiple selection

### Element UI version
2.12.0

### OS/Browsers version
MacOS / Chrome 79.0.3945.130

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/zqt7rk2b/

### Steps to reproduce
I try to restrict multiple for just three item.

I do this by vue's watch , with the data update, the Cascader still can add selection item.

```

var Main = {
data() {
return {
categoryIds: [],
jobCatetoryProps: {
multiple: true,
expandTrigger: "hover",
value: "id",
label: "name",
emitPath: false,
children: "children"
},
jobCategoryOptions: response().data
}
},
watch: {
categoryIds: function(newVal, oldVal) {
// limit 3 items, but not work
if (newVal.length > 3) {
this.categoryIds = oldVal;
}
},
}
}
```

### What is Expected?
the Cascader should restrict multiple selection for three item.

### What is actually happening?
the Cascader still can add selection item.