cpuidle: menu: Take negative "sleep length" values into account

Make the menu governor check the tick_nohz_get_next_hrtimer()
return value so as to avoid dealing with negative "sleep length"
values and make it use that value directly when the tick is stopped.

While at it, rename local variable delta_next in menu_select() to
delta_tick which better reflects its purpose.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>