net/mlx5: E-Switch, Reg/unreg function changed event at correct stage

When driver is doing eswitch mode change, it's critical to keep number
of enabled VFs unchanged. However, it can be changed on the fly once
function changed event is registered.

To remove this uncertainty, function changed event should not be
registered before all setups, and first be unregistered before all
cleanups. Wrap this functionality together with vport event handler.

Fixes: 61fc880839e6 ("net/mlx5: E-Switch, Handle representors creation in handler context")
Signed-off-by: Bodong Wang <bodong@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>