ethtool: report missing header via ext_ack in the default handler

The actual presence check for the header is in
ethnl_parse_header_dev_get() but it's a few layers in,
and already has a ton of arguments so let's just pick
the low hanging fruit and check for missing header in
the default request handler.

Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>