iio: core: Use min() instead of min_t() to make code more robust

min() has strict type checking and preferred over min_t() for
unsigned types to avoid overflow. Here it's unclear why min_t()
was chosen since both variables are of the same type. In any
case update to use min().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20230721170022.3461-5-andriy.shevchenko@linux.intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>