func_periodic_hook.c: Cleanup module resources on failure.

* Make load_module() cleanup if it failed to setup the module.

* Make unload_module() always return 0.  It is silly to fail unloading if
the hook function we try to unregister was not even registered.

Change-Id: I280fc6e8ba2a7ee2588ca01d870eebaf74b4ffe6