net: systemport: do not clear IFF_MULTICAST flag

The SYSTEMPORT Ethernet MAC supports multicast just fine, it just lacks
any sort of Unicast/Broadcast/Multicasting filtering at the Ethernet MAC
level since that is handled by the front end Ethernet switch, but that
is properly handled by bcm_sysport_set_rx_mode().

Some user-space applications might be relying on the presence of this
flag to prevent using multicast sockets, this also prevents that
interface from joining the IPv6 all-router mcast group.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>