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>