net: cosmetic: Un-typedef Ethernet_t

Separate the Ethernet header from the 802 header.
Base the size constants on the structs.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/net/arp.h b/net/arp.h
index b59fbdd..b7a1ac9 100644
--- a/net/arp.h
+++ b/net/arp.h
@@ -25,6 +25,6 @@
 void ArpInit(void);
 void ArpRequest(void);
 void ArpTimeoutCheck(void);
-void ArpReceive(Ethernet_t *et, struct ip_udp_hdr *ip, int len);
+void ArpReceive(struct ethernet_hdr *et, struct ip_udp_hdr *ip, int len);
 
 #endif /* __ARP_H__ */