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/include/net/wpan-phy.h b/include/net/wpan-phy.h
index 7b7fc58..f63537c 100644
--- a/include/net/wpan-phy.h
+++ b/include/net/wpan-phy.h
@@ -45,7 +45,11 @@
 };
 
 struct wpan_phy *wpan_phy_alloc(size_t priv_size);
-int wpan_phy_register(struct device *parent, struct wpan_phy *phy);
+static inline void wpan_phy_set_dev(struct wpan_phy *phy, struct device *dev)
+{
+	phy->dev.parent = dev;
+}
+int wpan_phy_register(struct wpan_phy *phy);
 void wpan_phy_unregister(struct wpan_phy *phy);
 void wpan_phy_free(struct wpan_phy *phy);
 /* Same semantics as for class_for_each_device */