ACPI: cleanup: make disable_acpi() valid w/o CONFIG_ACPI

Len Brown <lenb@kernel.org> said:
> Okay, but better to use disable_acpi()
> indeed, since this would be the first code not already inside CONFIG_ACPI
> to invoke disable_acpi(), we could define the inline as empty and you could
> then scratch the #ifdef too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Len Brown <len.brown@intel.com>