net/mlx5e: Warn when destroying mod hdr hash table that is not empty

To avoid memory leaks add a warn when destroying mod hdr hash table
but the hash table is not empty.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>