net: igb: convert to timecounter adjtime.

This patch changes the driver to use the new and improved method
for adjusting the offset of a timecounter.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>