iio: adc: rn5t618: Re-use generic struct u16_fract

Instead of custom data type re-use generic struct u16_fract.
No changes intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220126135353.24007-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>