intel_idle: disable module support

Right now the module capability is cauing more trouble
than it is worth.  At least one distro built intel_idle as a module
where it lost the init race with ACPI, making it useless.

Make intel_idle bool so that if you select it, you will use it.

We can restore module capability after cpuidle is enhanced
to handle run-time changing of idle drivers.

Signed-off-by: Len Brown <len.brown@intel.com>