fpga: dfl: make init callback optional

This patch makes init callback of sub features optional. With
this change, people don't need to prepare any empty init callback.

Signed-off-by: Wu Hao <[e-mail]>
Acked-by: Moritz Fischer <[e-mail]>
Signed-off-by: Moritz Fischer <[e-mail]>

drivers/fpga/dfl.c

···
 				     struct dfl_feature *feature,
 				     struct dfl_feature_driver *drv)
 {
-	int ret;
+	int ret = 0;
 
-	ret = drv->ops->init(pdev, feature);
-	if (ret)
-		return ret;
+	if (drv->ops->init) {
+		ret = drv->ops->init(pdev, feature);
+		if (ret)
+			return ret;
+	}
 
 	feature->ops = drv->ops;