igb: introduce ptp_flags variable and use it to replace IGB_FLAG_PTP

Upcoming patches will introduce new PTP specific flags. To avoid
cluttering the normal flags variable, introduce PTP specific "ptp_flags"
variable for this purpose, and move IGB_FLAG_PTP to become
IGB_PTP_ENABLED.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>