net/mlx5: simplify flow counters support check

net/mlx5: simplify flow counters support check

The redundant check of Flow counters support in runtime is removed.
The flag flow_counter_en is eliminated from the code. The Verbs
create counter function just returns an error if no counter
support presented in the system.

If there is no any of Flow counters configuration macro defined
the log message is emited, indicating the missing counter support.

mlx5_flow_validate_action_count() fuctnion is also updated due to
flow_counter_en flag removal.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>