IB/mlx5: Add necessary delay drop assignment

Assign the statistics and configuration structure pointer on success.

Fixes: fe248c3a ('IB/mlx5: Add delay drop configuration and statistics')
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>