tools/net/ynl: sanitise enums with leading digits in ynl-gen-c

Turn attribute names with leading digits into valid C names by
prepending an underscore, e.g. 5ghz -> _5ghz

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250211120127.84858-7-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>