ftgmac100: Add ndo_set_rx_mode() and support for multicast & promisc

This adds the ndo_set_rx_mode() callback to configure the
multicast filters, promisc and allmulti options.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>