igc: Complete to commit Add basic skeleton for PTP

igc: Complete to commit Add basic skeleton for PTP

commit d5794b1c61a9 ("igc: Add basic skeleton for PTP") added basic
support for PTP, what's missing is support for suspending.
Legacy power management has been added. Now we can add
the suspend method to the igc_shutdown.
By cleaning the runtime storage for timestamp this avoids a possible
invalid memory access when the system comes back from suspend state.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>