iio: humidity: hts221: refactor write_with_mask code

Move bit-shift in hts221_write_with_mask() instead of coding
the shift depth in the configured value. That change will be necessary
to fix an issue in device power-down procedure.
Simplify hts221_avg_list table management

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>