can: slcan: use the generic can_change_mtu()

It is useless to define a custom function that does nothing but always
return the same error code. Better to use the generic can_change_mtu()
function.
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Link: https://lore.kernel.org/all/20220728070254.267974-6-dario.binacchi@amarulasolutions.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/slcan/slcan-core.c

@@ -743,16 +743,11 @@ static int slcan_netdev_open(struct net_device *dev)
 	return err;
 }
 
-static int slcan_netdev_change_mtu(struct net_device *dev, int new_mtu)
-{
-	return -EINVAL;
-}
-
 static const struct net_device_ops slcan_netdev_ops = {
 	.ndo_open               = slcan_netdev_open,
 	.ndo_stop               = slcan_netdev_close,
 	.ndo_start_xmit         = slcan_netdev_xmit,
-	.ndo_change_mtu         = slcan_netdev_change_mtu,
+	.ndo_change_mtu         = can_change_mtu,
 };
 
 /******************************************