igc: make non-global functions static

Fix sparse warning:
drivers/net/ethernet/intel/igc/igc_ptp.c:512:6:
	warning: symbol 'igc_ptp_tx_work' was not declared. Should it be static?
drivers/net/ethernet/intel/igc/igc_ptp.c:644:6:
	warning: symbol 'igc_ptp_suspend' was not declared. Should it be static?
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NChen Zhou <chenzhou10@huawei.com>
Tested-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igc/igc_ptp.c

@@ -509,7 +509,7 @@ static void igc_ptp_tx_hwtstamp(struct igc_adapter *adapter)
  * This work function polls the TSYNCTXCTL valid bit to determine when a
  * timestamp has been taken for the current stored skb.
  */
-void igc_ptp_tx_work(struct work_struct *work)
+static void igc_ptp_tx_work(struct work_struct *work)
 {
 	struct igc_adapter *adapter = container_of(work, struct igc_adapter,
 						   ptp_tx_work);
@@ -641,7 +641,7 @@ void igc_ptp_init(struct igc_adapter *adapter)
  * This function stops the overflow check work and PTP Tx timestamp work, and
  * will prepare the device for OS suspend.
  */
-void igc_ptp_suspend(struct igc_adapter *adapter)
+static void igc_ptp_suspend(struct igc_adapter *adapter)
 {
 	if (!(adapter->ptp_flags & IGC_PTP_ENABLED))
 		return;