usbnet: make ethtool_ops const

The ethtool_ops table of function pointers should be const.
Fix all the usb network drivers.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/usb/sierra_net.c b/drivers/net/usb/sierra_net.c
index ed1b432..6253486 100644
--- a/drivers/net/usb/sierra_net.c
+++ b/drivers/net/usb/sierra_net.c
@@ -618,7 +618,7 @@
 	return sierra_net_get_private(dev)->link_up && netif_running(net);
 }
 
-static struct ethtool_ops sierra_net_ethtool_ops = {
+static const struct ethtool_ops sierra_net_ethtool_ops = {
 	.get_drvinfo = sierra_net_get_drvinfo,
 	.get_link = sierra_net_get_link,
 	.get_msglevel = usbnet_get_msglevel,