netkit: convert to ndo_set_rx_mode_async

Convert netkit driver from ndo_set_rx_mode to ndo_set_rx_mode_async.
The netkit driver's set_multicast_list is a no-op, presumably
for the same reason as the one in dummy? (fake multicast ability)

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260416185712.2155425-13-sdf@fomichev.me
Signed-off-by: Paolo Abeni <pabeni@redhat.com>