igc: Check __IGC_PTP_TX_IN_PROGRESS instead of ptp_tx_skb

The __IGC_PTP_TX_IN_PROGRESS flag indicates we have a pending Tx
timestamp. In some places, instead of checking that flag, we check
adapter->ptp_tx_skb. This patch fixes those places to use the flag.

Quick note about igc_ptp_tx_hwtstamp() change: when that function is
called, adapter->ptp_tx_skb is expected to be valid always so we
WARN_ON_ONCE() in case it is not.

Quick note about igc_ptp_suspend() change: when suspending, we don't
really need to check if there is a pending timestamp. We can simply
clear it unconditionally.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>