ixp4xx: Support the all multicast flag on the NPE devices.

This patch adds support for the IFF_ALLMULTI flag. Previously only the
IFF_PROMISC flag was supported.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Acked-By: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>