net: cdc_ncm: update datagram size after changing mtu

Current implementation updates the mtu size and notify cdc_ncm
device using USB_CDC_SET_MAX_DATAGRAM_SIZE request about datagram
size change instead of changing rx_urb_size.

Whenever mtu is being changed, datagram size should also be
updated. Also updating maxmtu formula so it takes max_datagram_size with
use of cdc_ncm_max_dgram_size() and not ctx.

Signed-off-by: Robert Dobrowolski <robert.dobrowolski@linux.intel.com>
Signed-off-by: Rafal Redzimski <rafal.f.redzimski@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/cdc_ncm.c

@@ -740,12 +740,14 @@ static void cdc_ncm_free(struct cdc_ncm_ctx *ctx)
 int cdc_ncm_change_mtu(struct net_device *net, int new_mtu)
 {
 	struct usbnet *dev = netdev_priv(net);
-	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
-	int maxmtu = ctx->max_datagram_size - cdc_ncm_eth_hlen(dev);
+	int maxmtu = cdc_ncm_max_dgram_size(dev) - cdc_ncm_eth_hlen(dev);
 
 	if (new_mtu <= 0 || new_mtu > maxmtu)
 		return -EINVAL;
+
 	net->mtu = new_mtu;
+	cdc_ncm_set_dgram_size(dev, new_mtu + cdc_ncm_eth_hlen(dev));
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(cdc_ncm_change_mtu);