tools: ynl: use ynl-gen -o instead of stdout in Makefile

tools: ynl: use ynl-gen -o instead of stdout in Makefile

Jiri added more careful handling of output of the code generator
to avoid wiping out existing files in
commit f65f305ae008 ("tools: ynl-gen: use temporary file for rendering")
Make use of the -o option in the Makefiles, it is already used
by ynl-regen.sh.

Link: https://lore.kernel.org/r/20231010202714.4045168-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>