etharp: cast netif_addr_idx_t to u16_t to match format type

This fixes an issue seen on MacOS with Clang 9.0.0:

../../../../lwip/src/core/ipv4/etharp.c:1069:142: error: format specifies type 'unsigned short' but the argument has type 'netif_addr_idx_t'
      (aka 'unsigned char') [-Werror,-Wformat]
        LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE, ("etharp_query: dropped previously queued packet %p for ARP entry %"U16_F"\n", (void *)q, i));
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~

../../../../lwip/src/core/ipv4/etharp.c:1074:120: error: format specifies type 'unsigned short' but the argument has type 'netif_addr_idx_t'
      (aka 'unsigned char') [-Werror,-Wformat]
      LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE, ("etharp_query: queued packet %p on ARP entry %"U16_F"\n", (void *)q, i));
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~

By default, netif_addr_idx_t is u8_t, so cast up to u16_t to match the format type

src/core/ipv4/etharp.c

@@ -1066,12 +1066,12 @@ etharp_query(struct netif *netif, const ip4_addr_t *ipaddr, struct pbuf *q)
 #else /* ARP_QUEUEING */
       /* always queue one packet per ARP request only, freeing a previously queued packet */
       if (arp_table[i].q != NULL) {
-        LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE, ("etharp_query: dropped previously queued packet %p for ARP entry %"U16_F"\n", (void *)q, i));
+        LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE, ("etharp_query: dropped previously queued packet %p for ARP entry %"U16_F"\n", (void *)q, (u16_t)i));
         pbuf_free(arp_table[i].q);
       }
       arp_table[i].q = p;
       result = ERR_OK;
-      LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE, ("etharp_query: queued packet %p on ARP entry %"U16_F"\n", (void *)q, i));
+      LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE, ("etharp_query: queued packet %p on ARP entry %"U16_F"\n", (void *)q, (u16_t)i));
 #endif /* ARP_QUEUEING */
     } else {
       ETHARP_STATS_INC(etharp.memerr);