igc: Refactor igc_ptp_set_timestamp_mode()

Current igc_ptp_set_timestamp_mode() logic is a bit tangled since it
handles many different hardware configurations in one single place,
making it harder to follow. This patch untangles that code by breaking
it into helper functions.

Quick note about the hw->mac.type check which was removed in this
refactoring: this check it not really needed since igc_i225 is the only
type supported by the IGC driver.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>