net/mlx4_en: Fix ip/udp steering rules multicast mac when attached via ethtool

Destination mac is a mandatory specification for ip/udp steering rules.
When attaching multicast steering rules via ethtool the unicast mac of the
interface was added to the rule specification instead of the multicast mac.
The following commit sets the corresponding multicast mac for the rule multicast ip.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>