igc: Don't reschedule ptp_tx work

The ptp_tx work is scheduled only if TSICR.TXTS bit is set, therefore
TSYNCTXCTL.TXTT_0 bit is expected to be set when we check it igc_ptp_tx_
work(). If it isn't, something is really off and rescheduling the ptp_tx
work to check it later doesn't help much. This patch changes the code to
WARN_ON_ONCE() if this situation ever happens.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>