Just setting up the periodic callback sampling time on initialization
may not work well for sensors that need to request for a sample with a
bus transaction, sleep and then read the new data. That's because the
function will be kept calling at a periodic rate, while the time in
which we can read the value is not really that sampling time, but rather
the time in which sensor was last read + the time spent in the function
before sending a new sample request.
Instead of creating a new type of thread to handle this case, just
implement the minimal and easy case of updating the period for this
callback, that can only be called from inside the callback function.
Peter Barker
Lucas De Marchi
Andrew Tridgell