iio: humidity: hts221: do not overwrite reserved data during power-down

GENMASK(6, 3) in CTRL_REG1 and GENMASK(6, 2) in CTRL_REG2 are marked
reserved. Do not changed the original value (not declared in the
datasheet) during power-down/suspend routines.

Fixes: e4a70e3e (iio: humidity: add support to hts221 rh/temp device)
Fixes: b7079eea (iio: humidity: hts221: add power management support)
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>