igb: Update PTP function names/variables and locations.

Where possible, move PTP-related functions into igb_ptp.c and update the
names of functions and variables to match the established coding style
in the files and specify that they are PTP-specific.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Jeff Pieper  <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>