tools: ynl-gen: fix single attribute structs with attr 0 only

tools: ynl-gen: fix single attribute structs with attr 0 only

Chuck run into an issue with a single-element attr-set which
only has an attr with value of 0. The search for max attr in
a struct records attrs with value larger than 0 only (max_val
is set to 0 at the start). Adjust the comparison, alternatively
max_val could be init'ed to -1. Somehow picking the last attr
of a value seems like a good idea in general.

Reported-by: Chuck Lever III <chuck.lever@oracle.com>
Fixes: be5bea1cc0bf ("net: add basic C code generators for Netlink")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>