forcedeth: multicast fix

This patch fixes the case where no multicast addresses are requested to
be added to the multicast filter. The multicast mask must be set to all
1's instead of all 0's.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>