m68k/macboing: Fix missed timer callback assignment

This fixes a missed function prototype callback from the timer conversions.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20171123221902.GA75727@beast