hello-mod/hello.c: convert to module_init/module_exit

Switch away from the old init_module/cleanup_module function names for the
main entry points. Change them to the documented method with module_init()
and module_exit() markers next to static functions.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>