igb: cleanup some of the code related to hw timestamping

The code for the hw timestamping is a bit bulky and making some of the
functions difficult to read.  In order to clean things up a bit I am moving
the timestamping operations into seperate functions.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>