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: Olliver Schinagl <oliver@schinagl.nl>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/include/net.h b/include/net.h
index c3bfb63..ed5259a 100644
--- a/include/net.h
+++ b/include/net.h
@@ -168,7 +168,8 @@
 
 #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;