Fixed some typo's in the comments.

src/netif/ethernetif.c

@@ -140,7 +140,7 @@ low_level_input(struct ethernetif *ethernetif)
        packet into the pbuf. */
     for(q = p; q != NULL; q = q->next) {
       /* Read enough bytes to fill this pbuf in the chain. The
-         avaliable data in the pbuf is given by the q->len
+         available data in the pbuf is given by the q->len
          variable. */
       read data into(q->payload, q->len);
     }