When using convert_frac to slow down a timer, the extension machinery is
used to prevent converting values that would be larger than, after
conversion, 2**32. In order to calculate the maximum value, UINT32_MAX
is converted.
Previously, this was mistakenly converted by multiplying with the
fraction used to scale up a value. E.g., when using convert_frac to
slow down a 1us timer to provide a 1ms timer, UINT32_MAX was multiplied
by 1000. As the result is used % UINT32_MAX, this lead to a wrong value
close to UINT32_MAX, as ((UINT32_MAX * N) % UINT32_MAX = (UINT32_MAX - N - 1)).
This PR fixes the calculation so max_value is determined by multiplying
with the fraction used to scale down a value.
