iio: adc: aspeed: Use model_data to set clk scaler.

This patch uses need_prescaler and scaler_bit_width to set the ADC clock
scaler.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210922081520.30580-5-billy_tsai@aspeedtech.com


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