net/mlx5: DR, Replace the check for valid STE entry

Validity check is done by reading the next lu_type from the STE,
this check can be replaced by checking the refcount.
This will make the check independent on internal STE structure.

Signed-off-by: Alex Vesker <valex@nvidia.com>
Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>