igb: refactor time sync interrupt handling

The code that handles the time sync interrupt is repeated in three
different places. This patch refactors the identical code blocks into
a single helper function.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>