Fix bug #50618: dhcp_remove_struct() macro does not work

Patch by Benjamin Silvestre

(cherry picked from commit 819224f0e53445d073985ded9fc2e8c2f6032928)

src/include/lwip/dhcp.h

@@ -108,7 +108,7 @@ struct dhcp
 
 void dhcp_set_struct(struct netif *netif, struct dhcp *dhcp);
 /** Remove a struct dhcp previously set to the netif using dhcp_set_struct() */
-#define dhcp_remove_struct(netif) do { (netif)->dhcp = NULL; } while(0)
+#define dhcp_remove_struct(netif) netif_set_client_data(netif, LWIP_NETIF_CLIENT_DATA_INDEX_DHCP, NULL)
 void dhcp_cleanup(struct netif *netif);
 err_t dhcp_start(struct netif *netif);
 err_t dhcp_renew(struct netif *netif);