net/mlx5e: Take advantage of the light-weight netdev open/stop

Now that TIRs, TISs and flow tables are kept alive while the netdev is
stopped (after executing ndo_stop()) we can do the following
improvements:

- Obsolete the active_vlans SW shadow.
- Do not delete/add flow table rules upon ndo_stop/open.
  In addition to simplifying the flow, this change also fastens
  the ndo_open/close operations.
- Obsolete synchronization of threads accessing the flow tables
  with the netdev stop/open threads.

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>