net: cdc_ncm: support rx_max/tx_max updates when running

Finish the rx_max/tx_max setup by flushing buffers and
informing usbnet about the changes.  This way, the settings
can be modified while the netdev is up and running.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>