net/mlx4_en: Disable HW clock overflow check when no HW support

Should not run HW clock overflow check if HW clock is not supported. Also, since
this watchdog is the only customer of service_task, no need to start it in that case.

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>