net/mlx5: Use own workqueue for lag netdev events processing

Instead of using the system workqueue, allocate our own workqueue.
This workqueue will be used to handle more work in the next patch.

This patch doesn't change functionality.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>