iavf: Implement settime64 with -EOPNOTSUPP

In the Linux kernel, the following vulnerability has been resolved: iavf: Implement settime64 with -EOPNOTSUPP ptp_clock_settime() assumes every ptp_clock has implemented settime64(). Stub it with -EOPNOTSUPP to prevent a NULL dereference. The fix is similar to commit 329d050bbe63 ("gve: Implement settime64 with -EOPNOTSUPP").