net/mlx4_en: Remove dependency between timestamping capability and service_task

Service task is responsible for other tasks in addition to timestamping
overflow check. Launch it even if timestamping is not supported by device.

Fixes: 07841f9d ('net/mlx4_en: Schedule napi when RX buffers allocation fails')
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>