net/mlx5: fix representor ID check for sampling

net/mlx5: fix representor ID check for sampling

The representor definition was introduced in the latest code.
For non-representor port, like PF port, use the 0xffff instead of -1.

This patch updates the representor id checking during splitting sample
flow.

Fixes: cb95feefdd03 ("net/mlx5: support sub-function representor")
Cc: stable@dpdk.org
Signed-off-by: Jiawei Wang <jiaweiw@nvidia.com>
Acked-by: Xueming Li <xuemingl@nvidia.com>