iio: adc: stm32-dfsdm: misc style improvements and fixes

Misc fixes & style improvements:
- checkpatch warns about line over 80 characters.
- remove extra spaces and a blank line (e.g. checkpatch --strict)
- remove bad error message always printed in probe routine.

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