net/mlx5: Bridge, Only handle registered netdev bridge events

Don't handle bridge events for a netdev that doesn't belong to
an eswitch that registered for bridge events.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>