netdev_ieee802154: add txpower to descriptor

drivers/include/net/netdev/ieee802154.h

@@ -114,6 +114,7 @@ typedef struct {
     uint8_t chan;                           /**< channel */
     uint8_t page;                           /**< channel page */
     uint16_t flags;                         /**< flags as defined above */
+    int16_t txpower;                        /**< tx power in dBm */
     /** @} */
 } netdev_ieee802154_t;