The integral accumulator of the PI servo is now a 64bit integer.
The stored values are bit shifted by 10, so we have a finer
granularity on the control.
With the previous implementation the integral part stopped working
when ofm was under the value of OPTS(ppi)->ai. This problem has
been solved with this changes.
I had to use __div64_32() from lib/div64.c, because dividing for
a long long is not allowed in arch-wrpc (it takes too much RAM).