mlx4_core: Reserve the correct number of MTT segments

Taking ilog2(dev->caps.reserved_mtts) to find out the order to pass to
the MTT buddy allocator will do the wrong thing if reserved_mtts is ever
not a power of 2.  Be safe and use fls(dev->caps.reserved_mtts - 1).

Signed-off-by: Roland Dreier <rolandd@cisco.com>