ethernet/ixp4xx: prevent allmulti from clobbering promisc

If both promisc and allmulti are set, promisc should trump allmulti and
disable the MAC filter; otherwise, the interface is not really promisc.

Previously, this code checked IFF_ALLMULTI prior to and without regard for
IFF_PROMISC; if both were set, only multicast and direct unicast traffic
would make it through the filter.

Signed-off-by: Derrick Pallas <pallas@meraki.com>
Signed-off-by: David S. Miller <davem@davemloft.net>