e1000e: factor out systim sanitization

This is prepatory work for an expanding list of adapter families that have
occasional ~10 hour clock jumps when being used for PTP. Factor out the
sanitization function and convert to using a feature (bug) flag, per
suggestion from Jesse Brandeburg.

Littering functional code with device-specific checks is much messier than
simply checking a flag, and having device-specific init set flags as needed.
There are probably a number of other cases in the e1000e code that
could/should be converted similarly.

Suggested-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>