mlx4_core: fix buffer overrun

When passing MLX4_UC_STEER=1 it was translated to value 2
after mlx4_QP_ATTACH_wrapper. Therefore in new_steering_entry()
unicast steer entries were added to index 2 of array of size 2.
Fixing this bug by shift right to one position.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.co.il>
Reviewed-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>