net: use core MTU range checking in USB NIC drivers

usbnet:
- Remove stale new_mtu <= 0 check in usbnet.c
- Set min_mtu = 0, max_mtu = 65535 (sub-drivers must set their own
  max_mtu and/or min_mtu as needed)

r8152:
- Set appropriate max_mtu for different variants (1500 or 9194)

lan78xx:
- Set max_mtu = 9000

asix_driver:
- max_mtu = 16384 for ax88178 variant

ax88179:
- max_mtu = 4088

cdc_ncm:
- max_mtu from hardware

cdc-phonet:
- min_mtu = 6, max_mtu = 65541

sierra_net:
- max_mtu = 1500, call usbnet_change_mtu directly
- sierra_net_change_mtu checked for MTU > 1500, then called
  usbnet_change_mtu, but if we set max_mtu to let the network core handle
  the range check, then we can simply call usbnet_change_mtu directly

smsc75xx:
- max_mtu = 9000

CC: netdev@vger.kernel.org
CC: Woojung Huh <woojung.huh@microchip.com>
CC: Microchip Linux Driver Support <UNGLinuxDriver@microchip.com>
CC: Hayes Wang <hayeswang@realtek.com>
CC: Oliver Neukum <oneukum@suse.com>
CC: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/usbnet.c

@@ -384,8 +384,6 @@ int usbnet_change_mtu (struct net_device *net, int new_mtu)
 	int		old_hard_mtu = dev->hard_mtu;
 	int		old_rx_urb_size = dev->rx_urb_size;
 
-	if (new_mtu <= 0)
-		return -EINVAL;
 	// no second zero-length packet read wanted after mtu-sized packets
 	if ((ll_mtu % dev->maxpacket) == 0)
 		return -EDOM;
@@ -1669,6 +1667,8 @@ usbnet_probe (struct usb_interface *udev, const struct usb_device_id *prod)
 	 * bind() should set rx_urb_size in that case.
 	 */
 	dev->hard_mtu = net->mtu + net->hard_header_len;
+	net->min_mtu = 0;
+	net->max_mtu = ETH_MAX_MTU;
 
 	net->netdev_ops = &usbnet_netdev_ops;
 	net->watchdog_timeo = TX_TIMEOUT_JIFFIES;