IB/mlx4: Remove redundant attribute in mlx4_ib_create_qp_rss struct

rx_key_len is not in use and needs to be removed.

Fixes: 3078f5f1 ("IB/mlx4: Add support for RSS QP")
Signed-off-by: Guy Levi <guyle@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>