igb: separate hardware setting from the set_ts_config ioctl

This patch separates the hardware logic from the set function, so that
we can re-use it during a ptp_reset. This enables the reset to return
functionality to the last known timestamp mode, rather than resetting
the value. We initialize the mode to off during the ptp_init cycle.

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>