ACPI: thermal: Adjust critical.flags.valid check

It is not necessary to compare critical.flags.valid to 1 in
acpi_thermal_trips_update() and doing so is also inconsistent with
other similar checks in that code, so simply check if the flag is
not 0 instead.

No expected functional impact.

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