wireless: don't write C files on failures

Change the scripting inside the shipped/extra certs C code
generation to not write the file when there are any failures.
That way, if the build aborts due to failures, we don't get
into a situation where a dummy file has been created and the
next build succeeds, but not with the desired output.

Fixes: 90a53e44

 ("cfg80211: implement regdb signature checking")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>