net/mlx5e: Enable napi in channel's activation stage

The channel's napi is first needed upon activation, not creation.
Minimize its enabled scope by moving it from the channel's open/close
stage into the activate/deactivate stage.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>