iio: gyro: st_gyro: fix L3GD20H support

Add proper support for L3GD20H gyroscope sensor. In particular:
- use L3GD20H as device name instead of L3GD20
- fix available full scales
- fix available sample frequencies

Note that the original patch listed first below introduced broken support for
this part.  The second patch drops the support as it didn't work.

This new patch brings in working support.

Fixes: 9444a300 (IIO: Add support for L3GD20H gyroscope)
Fixes: a0657716 ("iio:gyro: bug on L3GD20H gyroscope support")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>