cfg80211: fix callback type mismatches in wext-compat

cfg80211: fix callback type mismatches in wext-compat

Instead of casting callback functions to type iw_handler, which trips
indirect call checking with Clang's Control-Flow Integrity (CFI), add
stub functions with the correct function type for the callbacks.

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201117205902.405316-1-samitolvanen@google.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>