net/mlx5: Move legacy drop counter and rule under legacy structure

To improve code readability, move legacy drop counters and droup rule
under legacy structure.

While at it,
(a) prefix drop flow counters helper with legacy_.
(b) nullify the rule pointers only if they were valid.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>