resource/aws_opsworks_stack: Fixes for tfproviderlint R002 (#12028)

Reference: #9952

Remove pointer value dereferences, which can cause potential panics and are extraneous as `Set()` automatically handles pointer types including when `nil`.

Previously:

```
aws/resource_aws_opsworks_stack.go:328:38: R002: ResourceData.Set() pointer value dereference is extraneous
```

Output from acceptance testing:

```
--- PASS: TestAccAWSOpsworksStack_noVpcBasic (36.09s)
--- PASS: TestAccAWSOpsworksStack_noVpcCreateTags (46.83s)
--- PASS: TestAccAWSOpsworksStack_CustomCookbooks_SetPrivateProperties (40.27s)
--- PASS: TestAccAWSOpsworksStack_vpc (64.43s)
--- PASS: TestAccAWSOpsworksStack_noVpcChangeServiceRoleForceNew (43.03s)
```

aws/resource_aws_opsworks_stack.go

@@ -323,9 +323,9 @@ func resourceAwsOpsworksStackRead(d *schema.ResourceData, meta interface{}) erro
 			return dErr
 		}
 		// If the stack was found, set the stack_endpoint
-		if client.Config.Region != nil && *client.Config.Region != "" {
-			log.Printf("[DEBUG] Setting stack_endpoint for (%s) to (%s)", d.Id(), *client.Config.Region)
-			if err := d.Set("stack_endpoint", *client.Config.Region); err != nil {
+		if region := aws.StringValue(client.Config.Region); region != "" {
+			log.Printf("[DEBUG] Setting stack_endpoint for (%s) to (%s)", d.Id(), region)
+			if err := d.Set("stack_endpoint", region); err != nil {
 				log.Printf("[WARN] Error setting stack_endpoint: %s", err)
 			}
 		}