ACPICA: executer/exsystem: Add units to time variable names

ACPICA commit b69cbef7a83eadb102a1ff6c6f6fc5abce34805a

`how_long` refers to different units in both functions, so make it more
clear, what unit they expect. That also makes one comment superfluous.

Link: https://github.com/acpica/acpica/commit/b69cbef7


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