net: Print error message upon net usage when no ethernet-interface is found

This patch fixes a problem seen on PPC4xx boards, when no MAC address is
defined. Then no ethernet interface is available but a simple "tftp"
command will return without any error message which is quite confusing.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/net/eth.c b/net/eth.c
index 62297ab..16a6dcb 100644
--- a/net/eth.c
+++ b/net/eth.c
@@ -441,8 +441,10 @@
 {
 	struct eth_device* old_current;
 
-	if (!eth_current)
+	if (!eth_current) {
+		puts ("No ethernet found.\n");
 		return -1;
+	}
 
 	old_current = eth_current;
 	do {