cfg80211: always rewrite generated files from scratch

Currently the certs C code generation appends to the generated files,
which is most likely a leftover from commit 715a1233 ("wireless:
don't write C files on failures"). This causes duplicate code in the
generated files if the certificates have their timestamps modified
between builds and thereby trigger the generation rules.

Fixes: 715a1233 ("wireless: don't write C files on failures")
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>