net: fec: ptp: remove unused variable 'ns' in fec_time_keep()

Fixes the following W=1 kernel build warning(s):

drivers/net/ethernet/freescale/fec_ptp.c:523:6: warning:
 variable 'ns' set but not used [-Wunused-but-set-variable]
  523 |  u64 ns;
      |      ^~

After commit 6605b730 ("FEC: Add time stamping code and a PTP
hardware clock"), variable 'ns' is never used in fec_time_keep(),
so removing it to avoid build warning.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec_ptp.c

@@ -520,13 +520,12 @@ static void fec_time_keep(struct work_struct *work)
 {
 	struct delayed_work *dwork = to_delayed_work(work);
 	struct fec_enet_private *fep = container_of(dwork, struct fec_enet_private, time_keep);
-	u64 ns;
 	unsigned long flags;
 
 	mutex_lock(&fep->ptp_clk_mutex);
 	if (fep->ptp_clk_on) {
 		spin_lock_irqsave(&fep->tmreg_lock, flags);
-		ns = timecounter_read(&fep->tc);
+		timecounter_read(&fep->tc);
 		spin_unlock_irqrestore(&fep->tmreg_lock, flags);
 	}
 	mutex_unlock(&fep->ptp_clk_mutex);