MFC r331437:
Create designated workqueue for each mlx5en(4) device instance.

The mlx5e_destroy_ifp() function may be called from the system workqueue and
in this case trying to flush all works will cause a dead lock.
Instead of using the system workqueue, create a designated workqueue
for each mlx5en(4) device instance.

Submitted by: slavash@
Sponsored by: Mellanox Technologies