net: arc_emac: annonce IFF_MULTICAST support

Multicast support was implemented by commit 775dd682
('arc_emac: implement promiscuous mode and multicast filtering').

It can be enabled explicity using 'ifconfig eth0 multicast'.
The patch is needed in order to remove explicit configuration
as most devices has multicast mode enabled by default.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>