tools: ynl-gen: resolve enum vs struct name conflicts

Ethtool has an attribute set called stringset, from which
we'll generate struct ethtool_stringset. Unfortunately,
the old ethtool header declares enum ethtool_stringset
(the same name), to which compilers object.

This seems unavoidable. Check struct names against known
constants and append an underscore if conflict is detected.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>