net/mlx5e: Update NIC HW stats on demand only

Disable periodic stats update background thread and update stats in
background on demand when ndo_get_stats is called.

Having a background thread running in the driver all the time is bad for
power consumption and normally a user space daemon will query the stats
once every specific interval, so ideally the background thread and its
interval can be done in user space..

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>