net/mlx5e: Disable async events before unregister_netdev()

It does not make sense to allow events while the netdev is
unregistered.

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>