IB/mlx5: Don't set "block multicast loopback" capability

Currently Connect-IB does not support blocking multicast loopback, so
don't set IB_DEVICE_BLOCK_MULTICAST_LOOPBACK in the device caps.

Reported by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>