igc: Save PTP time before a reset

Many TSN features depend on the internal PTP clock, so the internal
PTP jumping when the adapter is reset can cause problems, usually in
the form of "TX Hangs" warnings in the driver.

The solution is to save the PTP time before a reset and restore it
after the reset is done. The value of the PTP time is saved before a
reset and we use the difference from CLOCK_MONOTONIC from reset time
to now, to correct what's going to be the new PTP time.

This is heavily inspired by commit bf4bf09b

 ("i40e: save PTP time
before a device reset").

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>