asus-wmi: Fix unused function build warning

Fix the following build warning

CC [M]  drivers/platform/x86/asus-wmi.o
drivers/platform/x86/asus-wmi.c:1356:13: warning: ‘do_nothing’ defined but not used [-Wunused-function]

Signed-off-by: Shuah Khan <shuah.khan@hp.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>