cpuidle: dt: Clarify a comment and simplify code in dt_init_idle_driver()

The drv->state_count is assigned the total number of available states, so
let's make that clear.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>