net: ftmac100: add multicast filtering possibility

If netdev_mc_count() is not zero and not IFF_ALLMULTI, filter
incoming multicast packets. The chip has a Multicast Address Hash Table
for allowed multicast addresses, so we fill it.

Implement .ndo_set_rx_mode and recalculate multicast hash table. Also
observe change of IFF_PROMISC and IFF_ALLMULTI netdev flags.

Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>