ACPI / PAD: Use time_before() for time comparison

To be future-proof and for better readability the time comparisons are
modified to use time_before() instead of plain, error-prone math.

Signed-off-by: Manuel Schölling <manuel.schoelling@gmx.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>