iio: adc: stm32-dfsdm: make spi_master_freq more accurate

Current ckout divider may be set to a value that makes ckout to exceed
spi-max-frequency. Rather use lower value (e.g. round up divider when
ckout isn't accurate).

Also when the SPI clock isn't accurate, 'spi_master_freq' is filled in
with expected frequency. Use computed value instead to be more accurate:
- e.g. source clock / (CKOUTDIV + 1)

Enforce checks on the divider: ckoutdiv range can be from 1-255 to provide
divider of 2-256.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>