gve: Implement settime64 with -EOPNOTSUPP

gve: Implement settime64 with -EOPNOTSUPP

ptp_clock_settime() assumes every ptp_clock has implemented settime64().
Stub it with -EOPNOTSUPP to prevent a NULL dereference.

Fixes: acd16380523b ("gve: Add initial PTP device support")
Reported-by: syzbot+a546141ca6d53b90aba3@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a546141ca6d53b90aba3
Signed-off-by: Tim Hostetler <thostet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Joshua Washington <joshwash@google.com>
Link: https://patch.msgid.link/20251029184555.3852952-3-joshwash@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>