Increase the number of QPs allowed per multicast group from 8 to 56.
This allows for one QP per core on 16-core systems, which are now
quite common, and allows some space for future growth.

This is basically the same patch that Jack Morgenstein
<jackm@dev.mellanox.co.il> just supplied for mlx4.
Signed-off-by: Roland Dreier <rolandd@cisco.com>