rndis_wlan: correct multicast_list handling V3

My previous patch (655ffee2) added locking in
a bad way. Because rndis_set_oid can sleep, there is need to prepare multicast
addresses into local buffer under netif_addr_lock first, then call
rndis_set_oid outside. This caused reorganizing of the whole function.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Reported-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>