net/mlx5: Fix creating a new FTE when an existing but full FTE exists

Currently, when a flow steering rule is added, we look for a FTE with
an identical value. If we find a match, we try to merge the required
destinations with the existing ones. In a case where the existing
destination list is full, the code should return an error to its
consumer. However, the current code just tries to create another FTE.
Fixing that by returning an error in this special scenario.

Fixes: f478be79a22e ("net/mlx5: Add hash table for flow groups in flow table")
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>