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

Modify the TEO governor to take possible negative return values of
tick_nohz_get_next_hrtimer() into account by changing the data type
of some variables used by it to s64 which allows it to carry out
computations without potentially problematic data type conversions
into u64.

Also change the computations in teo_select() so that the negative
values themselves are handled in a natural way to avoid adding extra
negative value checks to that function.

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