net/mlx5: E-switch, register event handler before arming the event

Currently, mlx5 is registering event handler for vport context change
event some time after arming the event. this can lead to missing an
event, which will result in wrong rules in the FDB.
Hence, register the event handler before arming the event.

This solution is valid since FW is sending vport context change event
only on vports which SW armed, and SW arming the vport when enabling
it, which is done after the FDB has been created.

Fixes: 6933a937 ("net/mlx5: E-Switch, Use async events chain")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>