igc: Complete to commit Add basic skeleton for PTP

commit 5f295805 ("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>