bonding: remove duplicate set of flag IFF_MULTICAST

Remove unnecessary set of flag IFF_MULTICAST, since ether_setup
already does this.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Andy Gospodarek <gospo@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>