net/cdc_ncm: Fix multicast RX support for CDC NCM devices with ZLP

net/cdc_ncm: Fix multicast RX support for CDC NCM devices with ZLP

ZLP for DisplayLink ethernet devices was enabled in 6.0:
932066cbbe11 ("net/cdc_ncm: Enable ZLP for DisplayLink ethernet devices").
The related driver_info should be the "same as cdc_ncm_info, but with
FLAG_SEND_ZLP". However, set_rx_mode that enables handling multicast
traffic was missing in the new cdc_ncm_zlp_info.

usbnet_cdc_update_filter rx mode was introduced in linux 5.9 with:
a5e7b76e6361 ("net: cdc_ncm: hook into set_rx_mode to admit multicast
traffic")

Without this hook, multicast, and then IPv6 SLAAC, is broken.

Fixes: 932066cbbe11 ("net/cdc_ncm: Enable ZLP for DisplayLink ethernet devices")
Signed-off-by: Santiago Ruano Rincón <santiago.ruano-rincon@imt-atlantique.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>