net: cdc_ncm: hook into set_rx_mode to admit multicast traffic

We set set_rx_mode to usbnet_cdc_update_filter provided
by cdc_ether that simply admits all multicast traffic
if there is more than one multicast filter configured.

Signed-off-by: Miguel Rodríguez Pérez <miguel@det.uvigo.gal>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>