ieee802154: add an mlme_ops call to retrieve PHY object

ops->get_phy should increment reference to wpan-phy. As we return
the external structure, we should do refcounting correctly.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>

include/net/wpan-phy.h

@@ -44,6 +44,8 @@ struct wpan_phy {
 	char priv[0] __attribute__((__aligned__(NETDEV_ALIGN)));
 };
 
+#define to_phy(_dev)	container_of(_dev, struct wpan_phy, dev)
+
 struct wpan_phy *wpan_phy_alloc(size_t priv_size);
 static inline void wpan_phy_set_dev(struct wpan_phy *phy, struct device *dev)
 {