Update ethernetif.c

if LWIP_NETIF_HOSTNAME enable, the length is 'sizeof(struct netif)+ LWIP_HOSTNAME_LEN',
not only  equal to  'sizeof(struct netif)'.

Author: DavidLin1577

components/net/lwip-2.0.2/src/netif/ethernetif.c

@@ -503,7 +503,11 @@ rt_err_t eth_device_init_with_flag(struct eth_device *dev, const char *name, rt_
         rt_kprintf("malloc netif failed\n");
         return -RT_ERROR;
     }
+#if LWIP_NETIF_HOSTNAME
+    rt_memset(netif, 0, sizeof(struct netif)+ LWIP_HOSTNAME_LEN);
+#else
     rt_memset(netif, 0, sizeof(struct netif));
+#endif
 
     /* set netif */
     dev->netif = netif;