iio: sensors-core: st: Allow full-scale to be an optional feature

Some chips either don't support it or fail to provide adequate documentation,
so sometimes it's impossible to enable the feature even if it is supported.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>