cpuidle: governors: menu: Rearrange main loop in menu_select()

Reduce the indentation level in the main loop of menu_select() by
rearranging some checks and assignments in it.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Link: https://patch.msgid.link/2389215.ElGaqSPkdT@rafael.j.wysocki