net: Get mac address from driver as seed

Previously seeded by obtaining mac addr from env. If mac addr was
never set, rand would output 0. This fix obtains the mac addr
from driver instead.
Signed-off-by: NJimmy Du <jimmy.du@ni.com>
Acked-by: NJoe Hershberger <joe.hershberger@ni.com>

net/net_rand.h

@@ -16,11 +16,11 @@
  */
 static inline unsigned int seed_mac(void)
 {
-	unsigned char enetaddr[6];
+	unsigned char enetaddr[ARP_HLEN];
 	unsigned int seed;
 
 	/* get our mac */
-	eth_getenv_enetaddr("ethaddr", enetaddr);
+	memcpy(enetaddr, eth_get_ethaddr(), ARP_HLEN);
 
 	seed = enetaddr[5];
 	seed ^= enetaddr[4] << 8;