ethtool: pse-pd: fix null-deref on genl_info in dump

ethnl_default_dump_one() passes NULL as info.

It's correct not to set extack during dump, as we should just
silently skip interfaces which can't provide the information.

Reported-by:  <syzbot+81c4b4bbba6eea2cfcae@syzkaller.appspotmail.com>
Fixes: 18ff0bcd ("ethtool: add interface to interact with Ethernet Power Equipment")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>