x86/idle: Cleanup idle_setup()

Updating the static call for x86_idle() from idle_setup() is
counter-intuitive.

Let select_idle_routine() handle it like the other idle choices, which
allows to simplify the idle selection later on.

While at it rewrite comments and return a proper error code and not -1.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240229142248.455616019@linutronix.de