ethtool: fix null-ptr-deref on ref tracker

dev can be a NULL here, not all requests set require_dev.

Fixes: e4b89540

 ("netlink: add net device refcount tracker to struct ethnl_req_info")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>