Fix a GCC warning due to unsigned substraction

Fix a GCC warning due to unsigned substraction

Given

    unsigned int reqRate, setRate, deviation;

then

    setRate - reqRate

is also an unsigned int - the code works "by chance" as `abs` casts that to an int which overflows and is technically undefined behaviour, for instance for setRate = 44100 and reqRate = 48000.

Thus cast them to int when computing the absolute value to ensure no UB.