drivers/net: const-ify ethtool_ops declarations

Signed-off-by: Jeff Garzik <jeff@garzik.org>
diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 4a9f40c..f429b19 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -190,7 +190,7 @@
 	return 1;
 }
 
-static struct ethtool_ops loopback_ethtool_ops = {
+static const struct ethtool_ops loopback_ethtool_ops = {
 	.get_link		= loopback_get_link,
 	.get_tso		= ethtool_op_get_tso,
 	.set_tso		= ethtool_op_set_tso,