iio:st_sensors: remove buffer allocation at each buffer enable

This patch is removing the buffer allocation at each buffer enable.
We just allocate enough memory in the main structure during probe
to cover maximum size needed (that anyway is pretty small) [16bytes].

Signed-off-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>