net/mlx4_core: Remove duplicate code line from procedure mlx4_bf_alloc

mlx4_bf_alloc had an unnecessary/duplicate code line. Did no harm,
but not good practice.

Reported by the Mellanox Beijing team.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>