net: fec: don't save PTP state if PTP is unsupported

In the Linux kernel, the following vulnerability has been resolved:

net: fec: don't save PTP state if PTP is unsupported

Some platforms (such as i.MX25 and i.MX27) do not support PTP, so on

these platforms fec_ptp_init() is not called and the related members

in fep are not initialized. However, fec_ptp_save_state() is called

unconditionally, which causes the kernel to panic. Therefore, add a

condition so that fec_ptp_save_state() is not called if PTP is not

supported.