[UML]: uml-net: don't set IFF_ALLMULTI in set_multicast_list

IFF_ALLMULTI is an indication from the network stack to the driver
to disable multicast filters, drivers should never set it directly.

Since the UML networking device doesn't have any filtering capabilites,
it doesn't the set_multicast_list function at all, it is kept so userspace
can still issue SIOCADDMULTI/SIOCDELMULTI ioctls however.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>