net: fec: remove stale comment

This comment is outdated as fec_ptp_ioctl has been replaced by fec_ptp_set/fec_ptp_get
since commit 1d5244d0

 ("fec: Implement the SIOCGHWTSTAMP ioctl")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>