net: micrel: Fix receiving the timestamp in the frame for lan8841

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

net: micrel: Fix receiving the timestamp in the frame for lan8841

The blamed commit started to use the ptp workqueue to get the second

part of the timestamp. And when the port was set down, then this

workqueue is stopped. But if the config option NETWORK_PHY_TIMESTAMPING

is not enabled, then the ptp_clock is not initialized so then it would

crash when it would try to access the delayed work.

So then basically by setting up and then down the port, it would crash.

The fix consists in checking if the ptp_clock is initialized and only

then cancel the delayed work.