net/mlx5: Tide up state_lock and vport enabled flag usage

When eswitch is disabled, vport event handler is unregistered.
This unregistration already synchronizes with running EQ event handler
in below code flow.

mlx5_eswitch_disable()
  mlx5_eswitch_event_handlers_unregister()
    mlx5_eq_notifier_unregister()
      atomic_notifier_chain_unregister()
        synchronize_rcu()

notifier_callchain
  eswitch_vport_event()
    queue_work()

Additionally vport->enabled flag is set under state_lock during
esw_enable_vport() but is not read under state_lock in
(a) esw_disable_vport() and (b) under atomic context
eswitch_vport_event().

It is also necessary to synchronize with already scheduled vport event.
This is already achieved using below sequence.

mlx5_eswitch_event_handlers_unregister()
  [..]
  flush_workqueue()

Hence,
(a) Remove vport->enabled check in eswitch_vport_event() which
doesn't make any sense.
(b) Remove redundant flush_workqueue() on every vport disable.
(c) Keep esw_disable_vport() symmetric with esw_enable_vport() for
state_lock.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Vu Pham <vuhuong@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>