sfc: Only cancel the PPS workqueue if it exists

The workqueue only exists for the primary PF. For other functions
we hit a WARN_ON in kernel/workqueue.c.

Fixes: 7c236c43 ("sfc: Add support for IEEE-1588 PTP")
Signed-off-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/ptp.c

@@ -1531,6 +1531,7 @@ void efx_ptp_remove(struct efx_nic *efx)
 	(void)efx_ptp_disable(efx);
 
 	cancel_work_sync(&efx->ptp_data->work);
+	if (efx->ptp_data->pps_workwq)
 		cancel_work_sync(&efx->ptp_data->pps_work);
 
 	skb_queue_purge(&efx->ptp_data->rxq);