mlx4_core: Allow log_mtts_per_seg up to 7

As there are now machines with a lot more physical memory, we need to
be able to register more memory. This patch lifts the upper limit of
log_mtts_per_seg from 5 to 7, increasing the amount of memory that can
be registered.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>