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/net/ieee802154/wpan-class.c b/net/ieee802154/wpan-class.c
index 0c51f85..cd42e88 100644
--- a/net/ieee802154/wpan-class.c
+++ b/net/ieee802154/wpan-class.c
@@ -168,10 +168,8 @@
 }
 EXPORT_SYMBOL(wpan_phy_alloc);
 
-int wpan_phy_register(struct device *parent, struct wpan_phy *phy)
+int wpan_phy_register(struct wpan_phy *phy)
 {
-	phy->dev.parent = parent;
-
 	return device_add(&phy->dev);
 }
 EXPORT_SYMBOL(wpan_phy_register);