igc: cleanup PTP module if probe fails

Make sure that the PTP module is cleaned up if the igc_probe() fails by
calling igc_ptp_stop() on exit.

Fixes: d89f8841 ("igc: Add skeletal frame for Intel(R) 2.5G Ethernet Controller support")
Signed-off-by: Christopher S M Hall <christopher.s.hall@intel.com>
Reviewed-by: Corinna Vinschen <vinschen@redhat.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>