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/asix.c b/drivers/net/usb/asix.c
index df2b08d..5857a40 100644
--- a/drivers/net/usb/asix.c
+++ b/drivers/net/usb/asix.c
@@ -1152,7 +1152,7 @@
 	return 0;
 }
 
-static struct ethtool_ops ax88178_ethtool_ops = {
+static const struct ethtool_ops ax88178_ethtool_ops = {
 	.get_drvinfo		= asix_get_drvinfo,
 	.get_link		= asix_get_link,
 	.get_msglevel		= usbnet_get_msglevel,