uapi: Convert some uses of 6 to ETH_ALEN

Use the #define where appropriate.

Add #include <linux/if_ether.h>
where appropriate too.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/dn.h

@@ -2,6 +2,7 @@
 #define _LINUX_DN_H
 
 #include <linux/types.h>
+#include <linux/if_ether.h>
 
 /*
 
@@ -120,7 +121,7 @@ struct linkinfo_dn {
  * Ethernet address format (for DECnet)
  */
 union etheraddress {
-        __u8 dne_addr[6];             /* Full ethernet address */
+        __u8 dne_addr[ETH_ALEN];      /* Full ethernet address */
   struct {
                 __u8 dne_hiord[4];    /* DECnet HIORD prefix   */
                 __u8 dne_nodeaddr[2]; /* DECnet node address   */