igb: restore PPS signal on igb_ptp_reset

When a reset occurs, the PPS SYS_WRAP interrupt was not re-enabled which
resulted in disabling of the PPS signaling. Fix this by recording when
the interrupt is on and ensuring that we re-enable it every time we
reset.

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>