net: cosmetic: Define ethernet name length

There are various places where the ethernet device name is defined to
several different sizes. Lets add a define and start using it.
Signed-off-by: NOlliver Schinagl <oliver@schinagl.nl>
Acked-by: NJoe Hershberger <joe.hershberger@ni.com>

include/net.h

@@ -168,7 +168,8 @@ void eth_halt_state_only(void); /* Set passive state */
 
 #ifndef CONFIG_DM_ETH
 struct eth_device {
-	char name[16];
+#define ETH_NAME_LEN 16
+	char name[ETH_NAME_LEN];
 	unsigned char enetaddr[ARP_HLEN];
 	phys_addr_t iobase;
 	int state;