doc: fix flow integrity hardware support in mlx5 guide

doc: fix flow integrity hardware support in mlx5 guide

Current MLX5 PMD documentation says that entire `ConnectX-6` family
supports flow integrity feature.

Flow integrity offload is not supported on vanilla `ConnectX-6`.
It is available on `ConnectX-6 Dx`, `ConnectX-6 Lx` and
`BlueField 2`.

Fixes: 79f8952783d0 ("net/mlx5: support integrity flow item")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>