iavf: Implement settime64 with -EOPNOTSUPP

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").

Fixes: d734223b2f0d ("iavf: add initial framework for registering PTP clock")
Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Tim Hostetler <thostet@google.com>
Link: https://patch.msgid.link/20251126094850.2842557-1-mschmidt@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>