ethtool: strset: report missing ETHTOOL_A_STRINGSET_ID via ext_ack

Strset needs ETHTOOL_A_STRINGSET_ID, use it as an example of
reporting attrs missing in nests.

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