igc: handle the IGC_PTP_ENABLED flag correctly

All functions in igc_ptp.c called from igc_main.c should check the
IGC_PTP_ENABLED flag. Adding check for this flag to stop and reset
functions.

Fixes: 5f295805 ("igc: Add basic skeleton for PTP")
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>