mlx4: Get rid of the mlx4_interface.activate callback

The mlx4_interface.activate callback was introduced in commit
79857cd3 ("net/mlx4: Postpone the registration of net_device"). It
dealt with a situation when a netdev notifier received a NETDEV_REGISTER
event for a new net_device created by mlx4_en but the same device was
not yet visible to mlx4_get_protocol_dev(). The callback can be removed
now that mlx4_get_protocol_dev() is gone.

Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Tested-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>