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/ping.c b/net/ping.c
index 8542e22..04a594c 100644
--- a/net/ping.c
+++ b/net/ping.c
@@ -98,7 +98,7 @@
 	ping_send();
 }
 
-void ping_receive(Ethernet_t *et, struct ip_udp_hdr *ip, int len)
+void ping_receive(struct ethernet_hdr *et, struct ip_udp_hdr *ip, int len)
 {
 	ICMP_t *icmph = (ICMP_t *)&(ip->udp_src);
 	IPaddr_t src_ip;