tools: ynl: fix render-max for flags definition

Properly manage render-max property for flags definition type
introducing mask value and setting it to (last_element << 1) - 1
instead of adding max value set to last_element + 1

Fixes: be5bea1c

 ("net: add basic C code generators for Netlink")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>