mlx4_core: fix mtt range deallocation

The mtt range was allocated in mtt units but deallocated
in segments. Among the rest, this caused crash during hotplug removal

Reported-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Marcel Apfelbaum <marcela@mellanox.co.il>
Reviewed-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Tested-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>