net/mlx4_en: Add a service task

Add a service task to run tasks that needed to be executed periodically.
Currently the only task is a watchdog to catch NIC clock overflow, to make
timestamping accurate.
Will move the statistics task into this framework in a later patch.

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