RDMA/mlx4: Add missed RSS hash inner header flag

Despite being advertised to user space application, the RSS inner
header flag was filtered by checks at the beginning of QP creation
routine.

Cc: <stable@vger.kernel.org> # 4.15
Fixes: 4d02ebd9 ("IB/mlx4: Fix RSS hash fields restrictions")
Fixes: 07d84f7b

 ("IB/mlx4: Add support to RSS hash for inner headers")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>