wpan-phy: follow usual patter of devices registration

Follow the usual pattern of devices registration by adding new function
(wpan_phy_set_dev) that sets child->parent relationship and removing
parent argument from wpan_phy_register call.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
diff --git a/drivers/ieee802154/fakehard.c b/drivers/ieee802154/fakehard.c
index f6f2afe..4ea93cc 100644
--- a/drivers/ieee802154/fakehard.c
+++ b/drivers/ieee802154/fakehard.c
@@ -372,11 +372,12 @@
 			goto out;
 	}
 
+	wpan_phy_set_dev(phy, &pdev->dev);
 	SET_NETDEV_DEV(dev, &phy->dev);
 
 	platform_set_drvdata(pdev, dev);
 
-	err = wpan_phy_register(&pdev->dev, phy);
+	err = wpan_phy_register(phy);
 	if (err)
 		goto out;