datastore: flatten option validation not confined to field it is defined on

**Client**

datastore

**Environment**

macOS

**Go Environment**

go1.17.2 darwin/amd64
cloud.google.com/go/datastore v1.6.0

**Code**

e.g.
```go
package main

import (
"context"
"fmt"
"time"

"cloud.google.com/go/datastore"
)

type CustomTime struct {
T time.Time
}

type B struct {
IDs []string
}

type S struct {
A CustomTime `datastore:",flatten"`
Bs []B
}

func main() {
ctx := context.Background()
c, err := datastore.NewClient(ctx, "my-project")
if err != nil {
panic(err)
}

key, err := c.Put(ctx, datastore.IncompleteKey("s", nil), &S{})
if err != nil {
panic(err)
}
fmt.Println(key)
}
```

**Expected behavior**

The struct is saved with the CustomTime field flattened to the property "A.T" and Bs as it normally would

**Actual behavior**

I receive the error `datastore: flattening nested structs leads to a slice of slices: field "IDs"`. However this field is not supposed to be flattened. If you switch the order of S's fields to have `Bs` first, then I no longer receive the error. Once the order is changed you can add any other struct fields after `A` that do not contain a slice of slices and they are correctly saved as non-flattened structs. This leads me to believe the validation for the flatten option is in error, not the actual flattening of fields.