IB/mlx5: Fix blue flame buffer size calculation

A blue flame register is comprised of two buffers of equal size.

Fixes: 5fe9dec0 ("IB/mlx5: Use blue flame register allocator in mlx5_ib")
Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Noa Osherovich <noaos@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>