submac: add TX power to retrievable netopts

Author: fabian18

drivers/netdev_ieee802154_submac/netdev_ieee802154_submac.c

@@ -59,6 +59,9 @@ static int _get(netdev_t *netdev, netopt_t opt, void *value, size_t max_len)
         case NETOPT_IEEE802154_PHY:
             *((uint8_t*) value) = ieee802154_get_phy_mode(submac);
             return 1;
+        case NETOPT_TX_POWER:
+            *((int16_t *)value) = netdev_submac->dev.txpower;
+            return sizeof(int16_t);
         default:
             break;
     }

sys/net/link_layer/ieee802154/submac.c

@@ -783,8 +783,9 @@ int ieee802154_set_phy_conf(ieee802154_submac_t *submac, const ieee802154_phy_co
 
     /* Go back to RX if needed */
     if (current_state == IEEE802154_FSM_STATE_RX) {
-        res = ieee802154_radio_set_rx(dev);
-        assert (res >= 0);
+        int rx = ieee802154_radio_set_rx(dev);
+        assert(rx >= 0);
+        (void)rx;
     }
 
     return res;