iio:temperature:mlx90632: Reduce number of equal calulcations

TAdut4 was calculated each iteration although it did not change. In light
of near future additions of the Extended range DSP calculations, this
function refactoring will help reduce unrelated changes in that series as
well as reduce the number of new functions needed.

Signed-off-by: Crt Mori <cmo@melexis.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200818213737.140613-2-cmo@melexis.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>