net/mlx5: Destroy pool->mutex

Destroy pool->mutex when we destroy the pool.

Fixes: c36326d3 ("net/mlx5: Round-Robin EQs over IRQs")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>