timer: fix wrap handling for x86 PIT

Issue #1106
timer: fix corner case if counter is 0

If counter drops to 0, the wrap flag is set also. That means we have actually
no wrap around. The patch avoids to add too much time to the elapsed time
variable.

Issue #1106
pit: no lock required in pit anymore

timer service calls platform timer nowadays solely from the vary same thread
 - the entrypoint of the timer service

Issue #1106