ACPI: thermal: Drop valid flag from struct acpi_thermal_trip

Notice that the valid flag in struct acpi_thermal_trip is in fact
redundant, because the temperature field of invalid trips is always
equal to THERMAL_TEMP_INVALID, so drop it from there and adjust the
code accordingly.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>