net/mlx5e: Propagate an internal event in case uplink netdev changes

Whenever uplink netdev is set/cleared, propagate newly introduced event
to inform notifier blocks netdev was added/removed.

Move the set() helper to core.c from header, introduce clear() and
netdev_added_event_replay() helpers. The last one is going to be called
from rdma driver, so export it.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>