IB/mlx4: Always use the correct port for mirrored multicast attachments

When attaching a QP to a multicast address in bonded mode, there was an
assumption that the port of the QP must be #1. This assumption isn't the
case under the flow which enables maximal usage of the physical ports.

Fix it by always checking the port of the original flow and create the
mirrored flow on the other port.

Fixes: c6215745 ('IB/mlx4: Load balance ports in port aggregation mode')
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>