net: update undefined ->ndo_change_mtu() comment

Update ->ndo_change_mtu() callback comment to remove text
about returning error in case of undefined callback. This
change makes the comment match the existing code behavior.
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -914,8 +914,7 @@ struct xfrmdev_ops {
  *
  * int (*ndo_change_mtu)(struct net_device *dev, int new_mtu);
  *	Called when a user wants to change the Maximum Transfer Unit
- *	of a device. If not defined, any request to change MTU will
- *	will return an error.
+ *	of a device.
  *
  * void (*ndo_tx_timeout)(struct net_device *dev);
  *	Callback used when the transmitter has not made any progress