resource/aws_athena_workgroup: Pass pointers directly to ResourceData.Set()
resource/aws_athena_workgroup: Pass pointers directly to ResourceData.Set()

Output from acceptance testing:

```
--- PASS: TestAccAWSAthenaWorkGroup_basic (13.65s)
--- PASS: TestAccAWSAthenaWorkGroup_disappears (22.50s)
--- PASS: TestAccAWSAthenaWorkGroup_Description (22.89s)
--- PASS: TestAccAWSAthenaWorkGroup_EnforceWorkgroupConfiguration (24.08s)
--- PASS: TestAccAWSAthenaWorkGroup_SseS3Encryption (24.58s)
--- PASS: TestAccAWSAthenaWorkGroup_Tags (28.98s)
--- PASS: TestAccAWSAthenaWorkGroup_PublishCloudWatchMetricsEnabled (30.44s)
--- PASS: TestAccAWSAthenaWorkGroup_BytesScannedCutoffPerQuery (36.83s)
--- PASS: TestAccAWSAthenaWorkGroup_KmsEncryption (56.71s)
--- PASS: TestAccAWSAthenaWorkGroup_OutputLocation (58.03s)
PASS
```

kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws