cpuidle: menu: Get rid of first_idx from menu_select()

Rearrange the code in menu_select() so that the loop over idle states
always starts from 0 and get rid of the first_idx variable.

While at it, add two empty lines to separate conditional statements
from one another.

No intentional behavior changes.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>