net/mlx5: SF: Fix memory leak of work item

Cited patch in the fixes tag missed to free the allocated work.
Fix it by freeing the work after work execution.

Fixes: f3196bb0 ("net/mlx5: Introduce vhca state event notifier")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>