IB/mlx4: Handle protocol field in multicast table

The newest device firmware stores IB vs. Ethernet protocol in two bits
in members_count field of multicast group table (0: Infiniband, 1:
Ethernet).  When changing the QP members count for a multicast group,
it important not to reset this information.  When calling multicast
attach first time, the protocol type should be specified.  In this
patch we always set it IB, but in the future we will handle Ethernet
too.  When looking for a QP, the protocol type shoud be checked too.

Signed-off-by: Aleksey Senin <alekseys@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>